docs(research): investigate Docker build caching options
Ranked recommendation: fix the docker system prune -af --volumes step first (it deletes BuildKit's build cache before every build, so no Dockerfile change can help until that stops), then split the apt-get install layer from the Godot/Blender download layer so the ~470s apt layer becomes cache-eligible across nightly runs. Registry cache and Gitea Actions' actions/cache don't fit this repo's single persistent runner / multi-GB nightly-changing layers. Research only, no code changes.
This commit is contained in:
@@ -0,0 +1,282 @@
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# Can this repo's Docker build be sped up, and does caching exist for it?
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Research triggered by the real-run timing gap: ~5 min locally vs **~16 min** on
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the actual runner
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(run [1493](https://git.arthurerlich.de/haylan/godot-ci/actions/runs/1493)),
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dominated by the single `apt-get install` layer (~470s, `build-essential`,
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`mingw-w64`, X11/audio/udev dev headers) and the Godot export
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templates + Blender downloads.
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## Verdict: **yes — but the fix is "stop deleting the cache", not "add a cache"**
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The runner (`runs-on: ubuntu-latest` in
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[`.gitea/workflows/publish.yml`](../../.gitea/workflows/publish.yml)) is a
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**self-hosted, persistent** [act_runner](https://gitea.com/gitea/act_runner)
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host, not an ephemeral GitHub-style VM — the workflow's own "Free up runner
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disk" step and its comment only make sense on a persistent box ("*the
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runner's disk fills up over successive runs until a push … is what finally
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fails*"; an ephemeral runner starts clean every time and would never
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accumulate anything to prune). That persistence is exactly what makes plain
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Docker build-cache (no registry, no extra tooling) viable here — and it's
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also exactly what today's `docker system prune -af --volumes` step throws
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away before every single build, per Docker's own docs: `system prune`'s
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default removal set explicitly includes *"Unused build cache"*, and `-a`
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widens that to "all unused build cache," not just dangling entries
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([docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)).
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So right now, **local BuildKit cache cannot survive from one nightly run to
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the next no matter how the Dockerfile is written** — the prune step deletes
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it every time, before the build that could have reused it even starts.
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Ranked recommendations:
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1. **Stop nuking the build cache in the prune step.** Replace
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`docker system prune -af --volumes` with targeted pruning that skips
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builder cache: `docker container prune -f && docker image prune -af &&
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docker volume prune -f`, plus a separate, bounded
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`docker builder prune -f --filter until=24h` (or `--keep-storage=<N>GB`)
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to cap build-cache growth without wiping cache that's still same-day
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reusable. This alone is what unlocks everything below — no Dockerfile
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change helps while the current step runs first. Near-zero risk: it still
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frees the disk that motivated the step, just not by deleting reusable
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build cache along with it.
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2. **Split the `apt-get install` layer from the Godot/Blender download
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layer in the Dockerfile**, apt first. Right now both live in one giant
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`RUN` chained with `&&` (`Dockerfile` lines 19–62), so a single-byte
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change anywhere in that chain — including the Godot/Blender URLs that
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*do* change nightly — invalidates the apt install too. BuildKit's cache
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is a linear, per-instruction, content-addressed chain: it reuses a layer
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only if every instruction up to and including it is byte-identical to a
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cached run (Docker's build-cache docs: cache is invalidated "from that
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point" the instant one instruction's content changes,
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[docs.docker.com/build/cache](https://docs.docker.com/build/cache/)).
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The apt package list changes essentially never; `GODOT_VERSION`/
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`BLENDER_URL` change most nights. Splitting them means the ~470s apt
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layer becomes a cache hit on every night that doesn't touch the
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Dockerfile itself, while the Godot/Blender layer keeps rebuilding (it has
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to — new content every time a version bumps). Combined with #1, this is
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the actual fix for the dominant cost in the real run.
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3. **Add `RUN --mount=type=cache` for apt's own directories**
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(`/var/cache/apt`, `/var/lib/apt`, `sharing=locked`) as a smaller
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supplement to #2 — it persists apt's downloaded `.deb` cache independent
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of the Dockerfile-instruction cache chain, so even a Dockerfile edit that
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invalidates the layer doesn't force re-downloading every `.deb` from
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Debian's mirrors, only re-running `dpkg` against the still-local cache
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([docs.docker.com/reference/dockerfile/#run---mounttypecache](https://docs.docker.com/reference/dockerfile/#run---mounttypecache)).
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Worth doing alongside #2 since it's a few lines, but it's a hedge against
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Dockerfile churn, not the main win — #2 already gets full cache hits on
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the common case (no Dockerfile change, only build-args change).
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4. **Skip registry-based cache (`--cache-to/--cache-from type=registry`).**
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Not worth it here. Registry cache exists to share a build cache *across
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machines that don't share disk* — ephemeral GitHub-hosted runners, fleets
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of self-hosted runners, etc.
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([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)).
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This repo has exactly one persistent runner with its own disk, so #1–#3
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already give it that same cache locally, for free, with no push/pull
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round-trip. Pushing a *separate* cache image on top would add push/pull
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time on every run for a benefit local cache already provides, and it's a
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bad fit for this specific image's shape: the Godot/Blender layers are
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multi-GB and, by design, different nearly every night (new versions),
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so `mode=max` would repeatedly re-push multi-GB "cache" blobs that are
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very unlikely to ever be pulled back down. There's also an open question
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of whether it would even work: Gitea's container registry only gained
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general OCI-artifact support in 1.24
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([go-gitea/gitea#25846](https://github.com/go-gitea/gitea/issues/25846),
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closed via PR #34666) — before that it rejected non-image manifests with
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`"unsupported: Schema version is not supported"`. This repo's Gitea
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instance version wasn't confirmed as part of this research, so pushing a
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buildx cache manifest (which uses custom OCI media types, distinct from
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a normal image manifest) is not guaranteed to work without checking that
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first.
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5. **Gitea Actions' `actions/cache` action is the wrong tool for this,
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noted for completeness.** Gitea Actions does support a GitHub-compatible
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`actions/cache@v3`/`v4` via a cache server that ships enabled by default
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in `act_runner`
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([about.gitea.com tutorial](https://about.gitea.com/resources/tutorials/enable-gitea-actions-cache-to-accelerate-cicd/);
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[docs.gitea.com/usage/actions/overview](https://docs.gitea.com/usage/actions/overview/)),
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plus a separate `RUNNER_TOOL_CACHE` mechanism. Both are key/value or
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directory caches meant for things like `node_modules`/pip wheels between
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workflow steps — not a Docker layer-cache mechanism, and not something
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`docker build`/`docker push` (what this workflow actually calls, see
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below) has any hook into. It doesn't apply to this problem; #1–#3 do.
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## 1. What the workflow and Dockerfile actually do today
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`.gitea/workflows/publish.yml` runs plain `docker build` and `docker push` —
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not `docker/build-push-action` or `docker buildx build` with any cache flags
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(lines 86–89, 96/105/109). `runs-on: ubuntu-latest` is a Gitea-side runner
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*label*, matched against whatever labels this instance's `act_runner` was
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registered with — Gitea's own runner docs describe labels as exactly this
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kind of local mapping (`ubuntu-latest:docker://...` style entries a runner
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operator configures, not a GitHub-hosted VM type)
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([search result summary of docs.gitea.com/usage/actions, act_runner label
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docs](https://docs.gitea.com/1.23/usage/actions/act-runner/); the specific
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`main_runner` name mentioned as a possible identifier for this instance's
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runner doesn't appear anywhere in this repo's tracked files — it would only
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show up in the Gitea Actions run UI, not the workflow YAML). The disk-fill
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problem the "Free up runner disk" step's own comment describes (*"nothing
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here ever pruned old ones, so the runner's disk fills up over successive
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runs"*) is only possible on a runner whose disk persists between runs —
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corroborated independently by act_runner's own architecture: its Docker
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executor keeps a long-lived host Docker daemon and reuses the image cache
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across job runs by default (act_runner documentation on the docker/dind
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executor flavors and idle-cleanup behavior — runner cleans up stale
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workspaces on an interval rather than starting from a fresh disk each job).
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The `Dockerfile`'s expensive work is one `RUN` chain (lines 19–62): a single
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`apt-get install` covering `build-essential`, `scons`, `mingw-w64`, and the
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X11/audio/udev dev headers, immediately followed — in the same layer, via
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`&&` — by the `curl`+`unzip` of Godot's editor and export templates and the
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`curl`+`tar` of the Blender tarball. Because it's one `RUN`, it's one cache
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entry: any change anywhere in it (including the Godot/Blender URLs, which
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change on essentially every nightly build since the workflow tracks
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newest-stable) invalidates the whole thing, apt install included.
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## 2. Docker/BuildKit's own caching mechanisms (docs.docker.com)
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- **Layer cache (default, local, free)**: BuildKit caches each instruction's
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result keyed on that instruction's content plus everything before it in
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the Dockerfile; a build on the same machine reuses it automatically with
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no extra flags, provided the cache wasn't evicted
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([docs.docker.com/build/cache](https://docs.docker.com/build/cache/)).
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This is what #1+#2 above unlock for this repo — it needs nothing but (a)
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not deleting it and (b) ordering the Dockerfile so the part that doesn't
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change (apt) comes before the part that does (downloads).
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- **`RUN --mount=type=cache`**: a *build cache mount*, separate from the
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layer cache — a directory that survives across builder invocations
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without itself being part of the cached layer, intended for package-
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manager caches like apt's `/var/cache/apt`/`/var/lib/apt`. Needs
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`sharing=locked` for apt specifically, since apt needs exclusive access to
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its own state
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([docs.docker.com/reference/dockerfile/#run---mounttypecache](https://docs.docker.com/reference/dockerfile/#run---mounttypecache)).
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Persists even when the layer cache above gets invalidated by a Dockerfile
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edit — a smaller, complementary win, not a replacement for #2's layer
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split.
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- **`--cache-to`/`--cache-from type=registry`**: exports/imports build
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cache to/from an OCI registry, as a separate artifact from the final
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image, specifically to let machines that don't share local disk share a
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cache (`mode=max` caches every stage, not just the final one — bigger
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push, more reuse potential)
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([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)).
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Not needed for a single persistent runner (see verdict §4).
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- **`BUILDKIT_INLINE_CACHE`/`type=inline`**: embeds cache metadata directly
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in the pushed image manifest instead of a separate cache artifact —
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Docker's own CI guidance is "in most cases you want to use the inline
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cache exporter" for simple cases, but it only supports `mode=min` (final
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stage only) versus registry cache's `min`/`max`
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([docs.docker.com/build/ci/github-actions/cache](https://docs.docker.com/build/ci/github-actions/cache/)).
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Same applicability caveat as registry cache: solves a multi-machine
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problem this repo doesn't have.
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- Docker Engine ≥23 makes BuildKit (via buildx) the default builder for
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plain `docker build` — *"Set Buildx and BuildKit as the default builder on
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Linux. Alias `docker build` to `docker buildx build`"*
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([docs.docker.com/engine/release-notes/23.0](https://docs.docker.com/engine/release-notes/23.0/)).
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So the workflow's existing plain `docker build` call (no `buildx` in the
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command) should already be getting BuildKit's layer cache today — it's
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the prune step deleting it that matters, not the build command needing to
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change to buildx.
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## 3. `docker system prune -af --volumes` vs `docker builder prune`
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Confirmed directly against Docker's CLI reference:
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- `docker system prune`'s default removal set is stopped containers, unused
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networks, dangling images, and **unused build cache**; `-a`/`--all`
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widens image removal to *all* unused images (not just dangling), and
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`--volumes` additionally removes unattached anonymous volumes
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([docs.docker.com/reference/cli/docker/system/prune](https://docs.docker.com/reference/cli/docker/system/prune/)).
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There's no flag on `system prune` to keep build cache while still pruning
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everything else — build cache removal isn't optional once you call it.
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- `docker builder prune` is the narrower, cache-only equivalent: `--all` to
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remove all unused cache (not just dangling), `--filter until=<duration>`
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to only remove cache older than a given age, `--keep-storage=<size>` to
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cap total cache size instead of clearing it outright
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([docs.docker.com/reference/cli/docker/builder/prune](https://docs.docker.com/reference/cli/docker/builder/prune/)).
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This is the concrete lever for recommendation #1: keep freeing the disk the
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workflow's own comment says filled up (old images, old anonymous volumes),
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but do it with `container prune`/`image prune -af`/`volume prune` instead of
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`system prune`, and bound the build cache separately with
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`docker builder prune -f --filter until=24h` (or a `--keep-storage` cap)
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rather than deleting all of it unconditionally before every build. A same-
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day rerun (e.g. `workflow_dispatch` shortly after the nightly cron) would
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then still get a cache hit on the apt layer; a multi-day-old apt cache entry
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still gets swept on the next age-filtered prune, so disk doesn't grow
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unbounded either.
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## 4. Layer-order restructuring compatibility
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Splitting `Dockerfile`'s one `RUN` into "apt install" then "Godot/Blender
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downloads" is a pure reorder/split — no semantic change, no new
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dependencies, and it's exactly what BuildKit's per-instruction cache keying
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is designed to reward (§2). Its payoff is entirely contingent on §3: with
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`docker system prune -af --volumes` still running first, splitting the
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layer changes nothing, because the reusable apt-layer cache entry is deleted
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moments before the build that would've hit it starts. The two
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recommendations are a pair, not independent options.
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## 5. Registry cache realism for this specific image
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Weighed directly against this repo's shape:
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- **Storage cost**: `mode=max` registry cache stores every stage as its own
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cache layer set in the registry, separate from the pushed image
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([docs.docker.com/build/cache/backends/registry](https://docs.docker.com/build/cache/backends/registry/)) —
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for an image whose expensive layers are multi-GB Godot export templates
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and a Blender tarball that both change nearly nightly, that's another
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near-full copy of those multi-GB layers pushed to Gitea's package registry
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on top of the image push that already happens, for cache entries with a
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short effective lifetime (superseded the next time Godot/Blender bump).
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- **Gitea registry support**: Gitea's container registry is OCI-compliant
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for standard image/Helm-chart manifests
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([docs.gitea.com/usage/packages/container](https://docs.gitea.com/usage/packages/container/)),
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but general OCI-*artifact* support (the category buildx's cache manifest
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falls into, using non-image media types) only landed in Gitea 1.24 via
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[go-gitea/gitea#25846](https://github.com/go-gitea/gitea/issues/25846) —
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prior to that, pushes of non-standard-image manifests failed with
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`501 unsupported: Schema version is not supported`. This repo's actual
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Gitea server version wasn't checked as part of this research, so this is
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a real "verify before use" gate, not just a style preference.
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- **Time saved**: none of it addresses the local-persistent-runner case
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this repo actually has — §2/§4's plain layer-cache fix gets the same or
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better result (apt-layer cache hits) for free, with no push/pull latency
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and no registry storage growth.
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Net: registry cache isn't wrong in general, it's solving a problem (cache
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sharing across machines without shared disk) this repo doesn't have, at a
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storage and reliability cost this repo's layer shapes make worse than
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typical.
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## Sources
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- `Dockerfile` — this repo's build, current single `RUN` chain (apt install
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+ Godot download/unpack + Blender download/unpack), fetched at HEAD
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- `.gitea/workflows/publish.yml` — `runs-on: ubuntu-latest`, plain
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`docker build`/`docker push` (no buildx/cache flags), the "Free up runner
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disk" step and its comment explaining *why* it prunes before every build
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- https://docs.docker.com/build/cache/ — BuildKit layer cache mechanics,
|
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invalidation model
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- https://docs.docker.com/reference/dockerfile/#run---mounttypecache —
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`RUN --mount=type=cache`, apt cache-mount example, `sharing=locked`
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- https://docs.docker.com/build/cache/backends/registry/ — `--cache-to`/
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`--cache-from type=registry`, `mode=min`/`max`
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- https://docs.docker.com/build/ci/github-actions/cache/ —
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`BUILDKIT_INLINE_CACHE`/`type=inline` vs registry cache tradeoffs
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- https://docs.docker.com/engine/release-notes/23.0/ — BuildKit/buildx as
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default `docker build` builder since Engine 23.0
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- https://docs.docker.com/reference/cli/docker/system/prune/ — `docker
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system prune` default removal set (incl. build cache), `-a`/`--volumes`
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- https://docs.docker.com/reference/cli/docker/builder/prune/ — `docker
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builder prune`, `--filter until=`, `--keep-storage`
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- https://docs.gitea.com/usage/actions/act-runner and
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https://docs.gitea.com/1.23/usage/actions/act-runner/ — act_runner
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labels, docker/dind executor persistence and idle cleanup behavior
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- https://about.gitea.com/resources/tutorials/enable-gitea-actions-cache-to-accelerate-cicd/
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and https://docs.gitea.com/usage/actions/overview/ — Gitea Actions'
|
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`actions/cache` support and default-enabled cache server
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- https://docs.gitea.com/usage/packages/container/ — Gitea's OCI-compliant
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container registry
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- 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
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||||
supported` error prior to that
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||||
- https://gitea.com/gitea/act_runner — act_runner project, docker executor
|
||||
Reference in New Issue
Block a user