diff --git a/docs/research/act-runner-volume-cleanup.md b/docs/research/act-runner-volume-cleanup.md
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+# Why act_runner's Docker volumes accumulate, and how to clean them up
+
+> **Scope note:** this is about the **host/runner infrastructure** for
+> git.arthurerlich.de's Gitea Actions (`act_runner` + the Docker daemon it
+> drives, in the docker-compose setup for that Gitea instance) — **not**
+> about this repo's own `Dockerfile`/published image. This repo's
+> `publish.yml` already runs `docker system prune -af --volumes` before its
+> build for the same underlying reason described here (unbounded volume/image
+> growth on the CI host), which is what prompted this research. No access to
+> the actual Gitea docker-compose file was available or used; every claim
+> below is grounded in act_runner's/Gitea's/Docker's own docs and source, or
+> flagged as inferred/secondary where it isn't.
+
+## Recommendations (ranked)
+
+1. **Schedule `docker system prune -af --volumes` on the runner host on a
+   recurring basis (cron or a scheduled Gitea Action), the same pattern this
+   repo's own `publish.yml` already uses before its build.** — **Proven
+   mechanism, standard mitigation.** No built-in act_runner/Gitea equivalent
+   exists (see §2); this is the documented, generic Docker-level fix and is
+   what community guidance for this exact runner-volume-leak class of
+   problem converges on. Prefer a filtered form if the host also runs
+   same-day rebuilds you don't want swept up:
+   `docker system prune -af --volumes --filter "until=24h"` (or
+   `docker volume prune --filter "until=24h"` for a volumes-only, less
+   aggressive sweep) — see §2 for the exact filter semantics.
+2. **Confirm/enable act_runner's `container.valid_volumes` allowlist and
+   review `container.options`/`container.network`, but do not expect them to
+   fix the leak** — **Proven from act_runner's own config reference (§1.2):
+   these control what containers are *allowed* to mount, not automatic
+   cleanup after a job ends.** Leaving `valid_volumes` wide open
+   (`- '**'`) doesn't itself cause more leakage than default, but tightening
+   it is good hygiene independent of the disk problem.
+3. **If DinD (Docker-in-Docker) mode is in use, that is very likely where
+   the bulk of the leaked volumes come from** — **inferred, but from a
+   documented, closely-analogous mechanism (§1), not confirmed against this
+   host's actual config** since no docker-compose file was available. DinD
+   needs its own persistent Docker data directory (commonly volume-backed),
+   and any job container that declares a `VOLUME` in its Dockerfile gets an
+   anonymous volume that Docker does not remove on `docker rm` unless the
+   container was started with `--rm` — a mechanism GitHub's own
+   `actions/runner` maintainers confirmed by design for the conceptually
+   identical GitHub Actions Docker executor (§1.3). act_runner's job/step
+   containers are created, started, stopped, and removed as separate Docker
+   API calls (not a single `docker run --rm`), which is the same shape of
+   gap.
+4. **A dedicated Linux user for act_runner (and rootless Docker specifically)
+   is a security/isolation improvement, not a fix for volume accumulation.**
+   — **Proven boundary, directly stated in act_runner's own docs (§3).**
+   Do it for blast-radius/`docker.sock`-exposure reasons; don't expect it to
+   change disk usage. Running the Docker daemon itself in **rootless mode**
+   changes *where* volumes live (per-user data root under
+   `$XDG_RUNTIME_DIR`/`~/.local/share/docker` instead of `/var/lib/docker`)
+   but Docker's own rootless docs describe no change to prune/cleanup
+   semantics — unused volumes still only go away via an explicit prune,
+   run as that user against that daemon.
+
+**Net: there is no act_runner/Gitea-side switch that stops the leak.** The
+fix is operational — a scheduled prune — same shape as the one already
+present in this repo's own `publish.yml`.
+
+---
+
+## 1. Why this happens
+
+### 1.1 act_runner's job execution model
+
+Per Gitea's own Act Runner documentation, act_runner supports two execution
+modes: **Docker container mode** ("it is recommended to run jobs in a
+docker container", requiring a running Docker daemon) and **host mode**
+(jobs run directly on the machine act_runner is on, no Docker involved).
+Docker mode is the default/recommended mode and is what a normal
+docker-compose Gitea+act_runner setup uses.
+([docs.gitea.com/1.23/usage/actions/act-runner](https://docs.gitea.com/1.23/usage/actions/act-runner/))
+
+In Docker mode, act_runner creates a container per job (and per
+service-container / step container as declared in the workflow), runs it,
+and removes it — as separate Docker API calls, not a single atomic
+`docker run --rm ...`. This matters because Docker's own volume-removal
+rule is tied to *how* a container is removed, not just that it's gone (see
+§1.3).
+
+### 1.2 act_runner's `container.*` config settings
+
+From act_runner's own `config.yaml` reference (fetched from
+`gitea.com/gitea/act_runner`'s example config,
+[gitea.com/gitea/act_runner — config.example.yaml](https://gitea.com/gitea/act_runner/raw/branch/main/internal/pkg/config/config.example.yaml)):
+
+- **`container.docker_host`** — overrides the Docker daemon act_runner talks
+  to. Empty (default) auto-detects; `-` auto-detects without mounting the
+  socket into job containers. Controls *which* daemon runs jobs, not
+  cleanup.
+- **`container.network`** — `host`, `bridge`, or a custom network name;
+  empty auto-creates a per-job network. Unrelated to volumes.
+- **`container.privileged`** — "Privileged mode is required for
+  Docker-in-Docker." This is the flag that turns on DinD-style job
+  execution.
+- **`container.options`** — free-form extra flags passed to container
+  creation (e.g. `--add-host=...`, or extra `--volume` mounts). Whatever a
+  workflow or admin puts here can itself add more bind mounts/volumes per
+  job, but is opt-in, not a default source of leakage.
+- **`container.valid_volumes`** — an **allowlist** (glob syntax) of
+  volumes/bind mounts job containers are permitted to mount; empty means
+  none permitted, `'**'` means any. This is an authorization control, not a
+  lifecycle/cleanup control — it limits what *can* be mounted, and doesn't
+  cause or prevent leftover anonymous volumes from images that declare
+  `VOLUME` internally.
+- **`container.bind_workdir`** — relevant specifically to DinD: "required
+  for Docker-in-Docker (DinD) setups when jobs use docker compose with bind
+  mounts (e.g. `.:/app`), as volume-based workspaces are not accessible
+  from the DinD daemon's filesystem." Confirms DinD's workspace handling is
+  a distinct code path from plain Docker mode.
+- **`workdir_cleanup_age` / `idle_cleanup_interval`** — these exist, but per
+  the config reference they clean up **act_runner's own workspace
+  directories** (stale bind-mounted task-workdir folders on the host
+  filesystem, and orphaned host-mode scratch dirs) — **not Docker volumes**.
+  There is no equivalent `container`-level setting for pruning Docker
+  volumes.
+
+None of act_runner's documented settings amount to "clean up Docker volumes
+after a job." The closest things (`workdir_cleanup_age`,
+`idle_cleanup_interval`) are scoped to filesystem workdirs, a different
+mechanism from Docker volume objects.
+
+### 1.3 Is DinD specifically the source of the leak, vs plain Docker mode?
+
+Two independent, additive causes are documented:
+
+1. **Any Docker execution mode (plain or DinD): anonymous volumes from
+   job/service containers leak by Docker's own removal semantics.** This is
+   not act_runner-specific — it's how Docker containers work. GitHub's own
+   `actions/runner` maintainers documented the identical failure mode for
+   GitHub Actions' (conceptually equivalent) Docker executor: *"When using
+   service containers with a `VOLUME` declaration in their Dockerfile, an
+   anonymous volume is automatically created with the container. At the end
+   of the workflow, the container is stopped and removed, but the anonymous
+   volumes it created stay around... The runner creates, starts, stops and
+   removes containers with separate commands, so passing `--rm` to `docker
+   create` has no effect on `docker remove`."*
+   ([actions/runner#1885](https://github.com/actions/runner/issues/1885) —
+   maintainer-confirmed root cause, not community speculation). act_runner's
+   job execution is architecturally the same shape (separate create/start/
+   stop/remove calls against the Docker API), so the same leak mechanism
+   applies to it by design, absent extra config — this is the strongest
+   available evidence, though it is evidence from GitHub's runner, applied
+   by analogy to act_runner rather than a direct act_runner maintainer
+   statement (no act_runner-specific issue making this exact claim was
+   found in this pass).
+2. **DinD adds its own persistent data directory on top.** A Docker-in-Docker
+   daemon needs its own `/var/lib/docker`-equivalent to store the images/
+   layers/volumes *of the jobs it runs*, and community DinD setups
+   routinely back that directory with a Docker volume so it survives
+   restarts of the DinD container itself (e.g. "`/var/lib/docker` cannot be
+   on AUFS, so it needs to be made a volume" — a long-standing operational
+   note about DinD from general Docker-in-Docker tooling, not Gitea-specific,
+   flagged here as secondary/community evidence). This is additive to (1):
+   DinD setups accumulate both the generic per-job anonymous-volume leak
+   *and* whatever grows inside DinD's own backing store (images, build
+   cache, and volumes belonging to the containers it spawns), which is a
+   second, larger accumulation surface than plain Docker mode.
+
+**Conclusion:** the generic anonymous-volume leak happens in plain Docker
+mode too (it's a Docker container-removal semantics issue, not a DinD-only
+bug), but DinD mode is very likely the bigger contributor on a real host,
+because it adds a whole second Docker data root that grows independently.
+Without the actual docker-compose file, it isn't possible to confirm from
+this pass alone whether git.arthurerlich.de's act_runner is configured for
+DinD (`container.privileged: true` set) or plain Docker mode.
+
+---
+
+## 2. How to clean up
+
+### 2.1 `docker volume prune` / `docker system prune` semantics (Docker's own docs)
+
+Per Docker's official pruning reference
+([docs.docker.com/engine/manage-resources/pruning](https://docs.docker.com/engine/manage-resources/pruning/),
+[docs.docker.com/reference/cli/docker/volume/prune](https://docs.docker.com/reference/cli/docker/volume/prune/),
+[docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)):
+
+- **`docker system prune`** (no flags) removes stopped containers, networks
+  not used by at least one container, dangling images, and unused build
+  cache. **Volumes are excluded by default.**
+- **`docker system prune --volumes`** (or `-a --volumes` to also sweep
+  non-dangling unused images) additionally removes "all volumes not used by
+  at least one container."
+- **`docker volume prune`** removes unused local volumes; by default this
+  means **anonymous** volumes not referenced by any container. `--all`
+  extends this to unused **named** volumes too.
+- **`docker volume prune --filter "until=<duration>"`** narrows the sweep to
+  volumes older than the given age (still only unused ones) — a way to keep
+  volumes from a job that finished minutes ago while still cleaning up
+  older leftovers.
+
+**Safety on a host mid-job:** all of the above operate only on resources
+**not referenced by any container, running or stopped**. A volume mounted
+into a currently-running job container is never touched. This makes it safe
+to run on a schedule against a live runner host without coordinating with
+in-flight jobs — the documented behavior is conservative by construction,
+not merely "usually fine in practice."
+
+### 2.2 Does act_runner or Gitea ship any built-in cleanup for this?
+
+No. Searched act_runner's config reference, Gitea's Actions/runner
+documentation, and the two most relevant issue threads found
+([go-gitea/gitea#31457](https://github.com/go-gitea/gitea/issues/31457),
+about `act_runner` manageability generally, and
+[gitea.com/gitea/runner#167](https://gitea.com/gitea/runner/issues/167) /
+[go-gitea/gitea#24438](https://github.com/go-gitea/gitea/issues/24438),
+about `docker.sock` exposure) — none describe a built-in Docker volume
+cleanup mechanism, cron-style prune config, or maintainer statement
+addressing this exact problem. The only cleanup knobs act_runner documents
+(`workdir_cleanup_age`, `idle_cleanup_interval`) are scoped to its own
+host-filesystem workdirs, not Docker volume objects (§1.2). This absence is
+itself informative: there is nothing to "turn on" here — a scheduled
+external prune is the only available lever.
+
+### 2.3 Is a scheduled `docker system prune -af --volumes` the standard approach?
+
+Yes, and it is corroborated by both this repo's own precedent and general
+community guidance:
+
+- This repo's `publish.yml` already runs `docker system prune -af --volumes`
+  before its build, for the same underlying class of problem (disk
+  exhaustion from accumulated Docker state on a CI runner) — direct
+  precedent already in this codebase.
+- General community guidance for self-hosted CI runners converges on the
+  same shape of fix: a scheduled/cron `docker system prune` (optionally
+  filtered by age) run on the runner host. This is consistent, secondary
+  evidence (blog posts / community write-ups, not a Docker or Gitea
+  standards document) but aligns with what Docker's own docs make safe
+  (§2.1).
+
+**A more targeted alternative** — `docker volume prune --filter "until=24h"`
+run on a schedule (e.g. nightly cron) — trades some aggressiveness for
+narrower scope: it only ever removes volumes, leaves stopped
+containers/networks/images alone, and skips anything from the last 24h in
+case a long-running or recently-finished job's volume is still wanted for
+debugging. This is a reasonable middle ground if a full `system prune -af`
+feels too broad for the runner host (e.g. if other non-CI Docker workloads
+share the machine); `docker system prune -af --volumes` is simpler and is
+what this repo already does elsewhere.
+
+---
+
+## 3. Dedicated user for the runner: security concern, not a cleanup mechanism
+
+### 3.1 act_runner's own guidance
+
+Gitea's Act Runner docs (systemd install instructions) recommend creating a
+dedicated unprivileged `act_runner` user, and note that adding it to the
+`docker` group (needed for Docker mode) **"effectively gives act_runner
+root access to the system"** — framed entirely as a privilege/blast-radius
+concern.
+([docs.gitea.com/1.23/usage/actions/act-runner](https://docs.gitea.com/1.23/usage/actions/act-runner/))
+
+This is echoed more sharply in Gitea's own tracker:
+[gitea.com/gitea/runner#167](https://gitea.com/gitea/runner/issues/167) /
+mirrored as [go-gitea/gitea#24438](https://github.com/go-gitea/gitea/issues/24438),
+titled *"Gitea Actions is HIGHLY insecure due to binding of docker.sock into
+all containers (= root on host)"* — the concern raised (and act_runner's own
+`examples/vm/rootless-docker.md` walkthrough, built specifically in
+response to this class of concern) is **container escape / arbitrary root
+on the host via `docker.sock` exposure to untrusted job code**, not disk
+usage.
+
+### 3.2 Rootless Docker / user-namespace remapping (Docker's own docs)
+
+Per Docker's official rootless-mode docs
+([docs.docker.com/engine/security/rootless](https://docs.docker.com/engine/security/rootless/)):
+rootless mode "lets you run the Docker daemon and containers as a non-root
+user to mitigate potential vulnerabilities in the daemon and the container
+runtime" — again, a security-isolation feature. The docs describe no
+special relationship to volume/disk accumulation or cleanup behavior.
+act_runner's own `rootless-docker.md` walkthrough
+([gitea.com/gitea/act_runner — examples/vm/rootless-docker.md](https://gitea.com/gitea/act_runner/raw/branch/main/examples/vm/rootless-docker.md))
+is, in its own words, exclusively about security/isolation setup (dedicated
+user, rootless daemon, systemd units) and makes no mention of volume
+cleanup or storage management at any point.
+
+Practically, rootless mode does shift *where* Docker's data root (and
+therefore its volumes) lives — per-user, typically under
+`$XDG_RUNTIME_DIR` / `~/.local/share/docker` rather than the system-wide
+`/var/lib/docker` — but this only changes the location being filled up, not
+whether it fills up. The same `docker volume prune` semantics (§2.1) still
+apply, run as that user against that user's daemon.
+
+### 3.3 Conclusion
+
+- **(a) Dedicated Linux user for the act_runner service itself:** purely a
+  blast-radius/permissions control (who can touch what if the runner or a
+  job container is compromised). No documented bearing on volume
+  accumulation.
+- **(b) Rootless Docker / userns-remap:** also purely a security-isolation
+  mechanism per Docker's own docs. It relocates where volumes are stored
+  (per-user vs system-wide) but does not change prune semantics or provide
+  any automatic cleanup — the volume-growth problem is orthogonal to this
+  and is not solved by adopting either (a) or (b).
+
+Both are worth doing for `docker.sock`-exposure/security reasons
+independent of this investigation, but neither should be expected to move
+the disk-usage needle — only a scheduled prune (§1 recommendation #1, §2.3)
+addresses that.
+
+---
+
+## Sources
+
+- act_runner config reference — [gitea.com/gitea/act_runner: config.example.yaml](https://gitea.com/gitea/act_runner/raw/branch/main/internal/pkg/config/config.example.yaml)
+- Gitea Act Runner docs — [docs.gitea.com/1.23/usage/actions/act-runner](https://docs.gitea.com/1.23/usage/actions/act-runner/)
+- act_runner rootless Docker walkthrough — [gitea.com/gitea/act_runner: examples/vm/rootless-docker.md](https://gitea.com/gitea/act_runner/raw/branch/main/examples/vm/rootless-docker.md)
+- Gitea Actions `docker.sock` security issue — [gitea.com/gitea/runner#167](https://gitea.com/gitea/runner/issues/167) / mirrored [go-gitea/gitea#24438](https://github.com/go-gitea/gitea/issues/24438)
+- act_runner manageability issue — [go-gitea/gitea#31457](https://github.com/go-gitea/gitea/issues/31457) (no maintainer disk/volume-cleanup discussion found in this thread)
+- Anonymous volume leak, maintainer-confirmed root cause (GitHub's `actions/runner`, conceptually equivalent Docker executor) — [actions/runner#1885](https://github.com/actions/runner/issues/1885)
+- Docker pruning reference — [docs.docker.com/engine/manage-resources/pruning](https://docs.docker.com/engine/manage-resources/pruning/)
+- `docker volume prune` reference — [docs.docker.com/reference/cli/docker/volume/prune](https://docs.docker.com/reference/cli/docker/volume/prune/)
+- `docker system prune` reference — [docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)
+- Docker rootless mode — [docs.docker.com/engine/security/rootless](https://docs.docker.com/engine/security/rootless/)
diff --git a/docs/research/build-caching.md b/docs/research/build-caching.md
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--- /dev/null
+++ b/docs/research/build-caching.md
@@ -0,0 +1,282 @@
+# Can this repo's Docker build be sped up, and does caching exist for it?
+
+Research triggered by the real-run timing gap: ~5 min locally vs **~16 min** on
+the actual runner
+(run [1493](https://git.arthurerlich.de/haylan/godot-ci/actions/runs/1493)),
+dominated by the single `apt-get install` layer (~470s, `build-essential`,
+`mingw-w64`, X11/audio/udev dev headers) and the Godot export
+templates + Blender downloads.
+
+## Verdict: **yes — but the fix is "stop deleting the cache", not "add a cache"**
+
+The runner (`runs-on: ubuntu-latest` in
+[`.gitea/workflows/publish.yml`](../../.gitea/workflows/publish.yml)) is a
+**self-hosted, persistent** [act_runner](https://gitea.com/gitea/act_runner)
+host, not an ephemeral GitHub-style VM — the workflow's own "Free up runner
+disk" step and its comment only make sense on a persistent box ("*the
+runner's disk fills up over successive runs until a push … is what finally
+fails*"; an ephemeral runner starts clean every time and would never
+accumulate anything to prune). That persistence is exactly what makes plain
+Docker build-cache (no registry, no extra tooling) viable here — and it's
+also exactly what today's `docker system prune -af --volumes` step throws
+away before every single build, per Docker's own docs: `system prune`'s
+default removal set explicitly includes *"Unused build cache"*, and `-a`
+widens that to "all unused build cache," not just dangling entries
+([docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)).
+So right now, **local BuildKit cache cannot survive from one nightly run to
+the next no matter how the Dockerfile is written** — the prune step deletes
+it every time, before the build that could have reused it even starts.
+
+Ranked recommendations:
+
+1. **Stop nuking the build cache in the prune step.** Replace
+   `docker system prune -af --volumes` with targeted pruning that skips
+   builder cache: `docker container prune -f && docker image prune -af &&
+   docker volume prune -f`, plus a separate, bounded
+   `docker builder prune -f --filter until=24h` (or `--keep-storage=<N>GB`)
+   to cap build-cache growth without wiping cache that's still same-day
+   reusable. This alone is what unlocks everything below — no Dockerfile
+   change helps while the current step runs first. Near-zero risk: it still
+   frees the disk that motivated the step, just not by deleting reusable
+   build cache along with it.
+2. **Split the `apt-get install` layer from the Godot/Blender download
+   layer in the Dockerfile**, apt first. Right now both live in one giant
+   `RUN` chained with `&&` (`Dockerfile` lines 19–62), so a single-byte
+   change anywhere in that chain — including the Godot/Blender URLs that
+   *do* change nightly — invalidates the apt install too. BuildKit's cache
+   is a linear, per-instruction, content-addressed chain: it reuses a layer
+   only if every instruction up to and including it is byte-identical to a
+   cached run (Docker's build-cache docs: cache is invalidated "from that
+   point" the instant one instruction's content changes,
+   [docs.docker.com/build/cache](https://docs.docker.com/build/cache/)).
+   The apt package list changes essentially never; `GODOT_VERSION`/
+   `BLENDER_URL` change most nights. Splitting them means the ~470s apt
+   layer becomes a cache hit on every night that doesn't touch the
+   Dockerfile itself, while the Godot/Blender layer keeps rebuilding (it has
+   to — new content every time a version bumps). Combined with #1, this is
+   the actual fix for the dominant cost in the real run.
+3. **Add `RUN --mount=type=cache` for apt's own directories**
+   (`/var/cache/apt`, `/var/lib/apt`, `sharing=locked`) as a smaller
+   supplement to #2 — it persists apt's downloaded `.deb` cache independent
+   of the Dockerfile-instruction cache chain, so even a Dockerfile edit that
+   invalidates the layer doesn't force re-downloading every `.deb` from
+   Debian's mirrors, only re-running `dpkg` against the still-local cache
+   ([docs.docker.com/reference/dockerfile/#run---mounttypecache](https://docs.docker.com/reference/dockerfile/#run---mounttypecache)).
+   Worth doing alongside #2 since it's a few lines, but it's a hedge against
+   Dockerfile churn, not the main win — #2 already gets full cache hits on
+   the common case (no Dockerfile change, only build-args change).
+4. **Skip registry-based cache (`--cache-to/--cache-from type=registry`).**
+   Not worth it here. Registry cache exists to share a build cache *across
+   machines that don't share disk* — ephemeral GitHub-hosted runners, fleets
+   of self-hosted runners, etc.
+   ([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)).
+   This repo has exactly one persistent runner with its own disk, so #1–#3
+   already give it that same cache locally, for free, with no push/pull
+   round-trip. Pushing a *separate* cache image on top would add push/pull
+   time on every run for a benefit local cache already provides, and it's a
+   bad fit for this specific image's shape: the Godot/Blender layers are
+   multi-GB and, by design, different nearly every night (new versions),
+   so `mode=max` would repeatedly re-push multi-GB "cache" blobs that are
+   very unlikely to ever be pulled back down. There's also an open question
+   of whether it would even work: Gitea's container registry only gained
+   general OCI-artifact support in 1.24
+   ([go-gitea/gitea#25846](https://github.com/go-gitea/gitea/issues/25846),
+   closed via PR #34666) — before that it rejected non-image manifests with
+   `"unsupported: Schema version is not supported"`. This repo's Gitea
+   instance version wasn't confirmed as part of this research, so pushing a
+   buildx cache manifest (which uses custom OCI media types, distinct from
+   a normal image manifest) is not guaranteed to work without checking that
+   first.
+5. **Gitea Actions' `actions/cache` action is the wrong tool for this,
+   noted for completeness.** Gitea Actions does support a GitHub-compatible
+   `actions/cache@v3`/`v4` via a cache server that ships enabled by default
+   in `act_runner`
+   ([about.gitea.com tutorial](https://about.gitea.com/resources/tutorials/enable-gitea-actions-cache-to-accelerate-cicd/);
+   [docs.gitea.com/usage/actions/overview](https://docs.gitea.com/usage/actions/overview/)),
+   plus a separate `RUNNER_TOOL_CACHE` mechanism. Both are key/value or
+   directory caches meant for things like `node_modules`/pip wheels between
+   workflow steps — not a Docker layer-cache mechanism, and not something
+   `docker build`/`docker push` (what this workflow actually calls, see
+   below) has any hook into. It doesn't apply to this problem; #1–#3 do.
+
+## 1. What the workflow and Dockerfile actually do today
+
+`.gitea/workflows/publish.yml` runs plain `docker build` and `docker push` —
+not `docker/build-push-action` or `docker buildx build` with any cache flags
+(lines 86–89, 96/105/109). `runs-on: ubuntu-latest` is a Gitea-side runner
+*label*, matched against whatever labels this instance's `act_runner` was
+registered with — Gitea's own runner docs describe labels as exactly this
+kind of local mapping (`ubuntu-latest:docker://...` style entries a runner
+operator configures, not a GitHub-hosted VM type)
+([search result summary of docs.gitea.com/usage/actions, act_runner label
+docs](https://docs.gitea.com/1.23/usage/actions/act-runner/); the specific
+`main_runner` name mentioned as a possible identifier for this instance's
+runner doesn't appear anywhere in this repo's tracked files — it would only
+show up in the Gitea Actions run UI, not the workflow YAML). The disk-fill
+problem the "Free up runner disk" step's own comment describes (*"nothing
+here ever pruned old ones, so the runner's disk fills up over successive
+runs"*) is only possible on a runner whose disk persists between runs —
+corroborated independently by act_runner's own architecture: its Docker
+executor keeps a long-lived host Docker daemon and reuses the image cache
+across job runs by default (act_runner documentation on the docker/dind
+executor flavors and idle-cleanup behavior — runner cleans up stale
+workspaces on an interval rather than starting from a fresh disk each job).
+
+The `Dockerfile`'s expensive work is one `RUN` chain (lines 19–62): a single
+`apt-get install` covering `build-essential`, `scons`, `mingw-w64`, and the
+X11/audio/udev dev headers, immediately followed — in the same layer, via
+`&&` — by the `curl`+`unzip` of Godot's editor and export templates and the
+`curl`+`tar` of the Blender tarball. Because it's one `RUN`, it's one cache
+entry: any change anywhere in it (including the Godot/Blender URLs, which
+change on essentially every nightly build since the workflow tracks
+newest-stable) invalidates the whole thing, apt install included.
+
+## 2. Docker/BuildKit's own caching mechanisms (docs.docker.com)
+
+- **Layer cache (default, local, free)**: BuildKit caches each instruction's
+  result keyed on that instruction's content plus everything before it in
+  the Dockerfile; a build on the same machine reuses it automatically with
+  no extra flags, provided the cache wasn't evicted
+  ([docs.docker.com/build/cache](https://docs.docker.com/build/cache/)).
+  This is what #1+#2 above unlock for this repo — it needs nothing but (a)
+  not deleting it and (b) ordering the Dockerfile so the part that doesn't
+  change (apt) comes before the part that does (downloads).
+- **`RUN --mount=type=cache`**: a *build cache mount*, separate from the
+  layer cache — a directory that survives across builder invocations
+  without itself being part of the cached layer, intended for package-
+  manager caches like apt's `/var/cache/apt`/`/var/lib/apt`. Needs
+  `sharing=locked` for apt specifically, since apt needs exclusive access to
+  its own state
+  ([docs.docker.com/reference/dockerfile/#run---mounttypecache](https://docs.docker.com/reference/dockerfile/#run---mounttypecache)).
+  Persists even when the layer cache above gets invalidated by a Dockerfile
+  edit — a smaller, complementary win, not a replacement for #2's layer
+  split.
+- **`--cache-to`/`--cache-from type=registry`**: exports/imports build
+  cache to/from an OCI registry, as a separate artifact from the final
+  image, specifically to let machines that don't share local disk share a
+  cache (`mode=max` caches every stage, not just the final one — bigger
+  push, more reuse potential)
+  ([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)).
+  Not needed for a single persistent runner (see verdict §4).
+- **`BUILDKIT_INLINE_CACHE`/`type=inline`**: embeds cache metadata directly
+  in the pushed image manifest instead of a separate cache artifact —
+  Docker's own CI guidance is "in most cases you want to use the inline
+  cache exporter" for simple cases, but it only supports `mode=min` (final
+  stage only) versus registry cache's `min`/`max`
+  ([docs.docker.com/build/ci/github-actions/cache](https://docs.docker.com/build/ci/github-actions/cache/)).
+  Same applicability caveat as registry cache: solves a multi-machine
+  problem this repo doesn't have.
+- Docker Engine ≥23 makes BuildKit (via buildx) the default builder for
+  plain `docker build` — *"Set Buildx and BuildKit as the default builder on
+  Linux. Alias `docker build` to `docker buildx build`"*
+  ([docs.docker.com/engine/release-notes/23.0](https://docs.docker.com/engine/release-notes/23.0/)).
+  So the workflow's existing plain `docker build` call (no `buildx` in the
+  command) should already be getting BuildKit's layer cache today — it's
+  the prune step deleting it that matters, not the build command needing to
+  change to buildx.
+
+## 3. `docker system prune -af --volumes` vs `docker builder prune`
+
+Confirmed directly against Docker's CLI reference:
+
+- `docker system prune`'s default removal set is stopped containers, unused
+  networks, dangling images, and **unused build cache**; `-a`/`--all`
+  widens image removal to *all* unused images (not just dangling), and
+  `--volumes` additionally removes unattached anonymous volumes
+  ([docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)).
+  There's no flag on `system prune` to keep build cache while still pruning
+  everything else — build cache removal isn't optional once you call it.
+- `docker builder prune` is the narrower, cache-only equivalent: `--all` to
+  remove all unused cache (not just dangling), `--filter until=<duration>`
+  to only remove cache older than a given age, `--keep-storage=<size>` to
+  cap total cache size instead of clearing it outright
+  ([docs.docker.com/reference/cli/docker/builder/prune](https://docs.docker.com/reference/cli/docker/builder/prune/)).
+
+This is the concrete lever for recommendation #1: keep freeing the disk the
+workflow's own comment says filled up (old images, old anonymous volumes),
+but do it with `container prune`/`image prune -af`/`volume prune` instead of
+`system prune`, and bound the build cache separately with
+`docker builder prune -f --filter until=24h` (or a `--keep-storage` cap)
+rather than deleting all of it unconditionally before every build. A same-
+day rerun (e.g. `workflow_dispatch` shortly after the nightly cron) would
+then still get a cache hit on the apt layer; a multi-day-old apt cache entry
+still gets swept on the next age-filtered prune, so disk doesn't grow
+unbounded either.
+
+## 4. Layer-order restructuring compatibility
+
+Splitting `Dockerfile`'s one `RUN` into "apt install" then "Godot/Blender
+downloads" is a pure reorder/split — no semantic change, no new
+dependencies, and it's exactly what BuildKit's per-instruction cache keying
+is designed to reward (§2). Its payoff is entirely contingent on §3: with
+`docker system prune -af --volumes` still running first, splitting the
+layer changes nothing, because the reusable apt-layer cache entry is deleted
+moments before the build that would've hit it starts. The two
+recommendations are a pair, not independent options.
+
+## 5. Registry cache realism for this specific image
+
+Weighed directly against this repo's shape:
+
+- **Storage cost**: `mode=max` registry cache stores every stage as its own
+  cache layer set in the registry, separate from the pushed image
+  ([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)) —
+  for an image whose expensive layers are multi-GB Godot export templates
+  and a Blender tarball that both change nearly nightly, that's another
+  near-full copy of those multi-GB layers pushed to Gitea's package registry
+  on top of the image push that already happens, for cache entries with a
+  short effective lifetime (superseded the next time Godot/Blender bump).
+- **Gitea registry support**: Gitea's container registry is OCI-compliant
+  for standard image/Helm-chart manifests
+  ([docs.gitea.com/usage/packages/container](https://docs.gitea.com/usage/packages/container/)),
+  but general OCI-*artifact* support (the category buildx's cache manifest
+  falls into, using non-image media types) only landed in Gitea 1.24 via
+  [go-gitea/gitea#25846](https://github.com/go-gitea/gitea/issues/25846) —
+  prior to that, pushes of non-standard-image manifests failed with
+  `501 unsupported: Schema version is not supported`. This repo's actual
+  Gitea server version wasn't checked as part of this research, so this is
+  a real "verify before use" gate, not just a style preference.
+- **Time saved**: none of it addresses the local-persistent-runner case
+  this repo actually has — §2/§4's plain layer-cache fix gets the same or
+  better result (apt-layer cache hits) for free, with no push/pull latency
+  and no registry storage growth.
+
+Net: registry cache isn't wrong in general, it's solving a problem (cache
+sharing across machines without shared disk) this repo doesn't have, at a
+storage and reliability cost this repo's layer shapes make worse than
+typical.
+
+## Sources
+
+- `Dockerfile` — this repo's build, current single `RUN` chain (apt install
+  + Godot download/unpack + Blender download/unpack), fetched at HEAD
+- `.gitea/workflows/publish.yml` — `runs-on: ubuntu-latest`, plain
+  `docker build`/`docker push` (no buildx/cache flags), the "Free up runner
+  disk" step and its comment explaining *why* it prunes before every build
+- https://docs.docker.com/build/cache/ — BuildKit layer cache mechanics,
+  invalidation model
+- https://docs.docker.com/reference/dockerfile/#run---mounttypecache —
+  `RUN --mount=type=cache`, apt cache-mount example, `sharing=locked`
+- https://docs.docker.com/build/cache/backends/registry/ — `--cache-to`/
+  `--cache-from type=registry`, `mode=min`/`max`
+- https://docs.docker.com/build/ci/github-actions/cache/ —
+  `BUILDKIT_INLINE_CACHE`/`type=inline` vs registry cache tradeoffs
+- https://docs.docker.com/engine/release-notes/23.0/ — BuildKit/buildx as
+  default `docker build` builder since Engine 23.0
+- https://docs.docker.com/reference/cli/docker/system/prune/ — `docker
+  system prune` default removal set (incl. build cache), `-a`/`--volumes`
+- https://docs.docker.com/reference/cli/docker/builder/prune/ — `docker
+  builder prune`, `--filter until=`, `--keep-storage`
+- https://docs.gitea.com/usage/actions/act-runner and
+  https://docs.gitea.com/1.23/usage/actions/act-runner/ — act_runner
+  labels, docker/dind executor persistence and idle cleanup behavior
+- https://about.gitea.com/resources/tutorials/enable-gitea-actions-cache-to-accelerate-cicd/
+  and https://docs.gitea.com/usage/actions/overview/ — Gitea Actions'
+  `actions/cache` support and default-enabled cache server
+- https://docs.gitea.com/usage/packages/container/ — Gitea's OCI-compliant
+  container registry
+- https://github.com/go-gitea/gitea/issues/25846 (closed via PR #34666) —
+  Gitea container registry's OCI-artifact (non-image-manifest) support
+  landing in 1.24, and the `501 unsupported: Schema version is not
+  supported` error prior to that
+- https://gitea.com/gitea/act_runner — act_runner project, docker executor