feat: add custom godot-ci export-toolchain image

Bakes export-template.yml's apt-get toolchain into an image extending
barichello/godot-ci:4.7.1, plus a publish workflow to push it to this
repo's Gitea package registry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

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+# godot-ci
+
+Custom image extending `barichello/godot-ci:4.7.1` with the Godot 4.7.1 export-template toolchain
+pre-installed, so `Project4x`'s `export-template.yml` doesn't `apt-get install` it on every release
+run.
+
+Built from the handoff spec at `Project4x`'s `docs/handoff/godot-ci-custom-image.md`.
+
+## Image
+
+`git.arthurerlich.de/haylan/godot-ci:4.7.1`
+
+Published automatically by `.gitea/workflows/publish.yml` on every push to `main` that touches
+`Dockerfile`, or manually via workflow dispatch.
+
+A Godot version bump means updating `Dockerfile`'s `FROM` line and the tag in both the workflow
+and this README together.
+
+## Auth
+
+The publish workflow pushes to this repo's Gitea package (container) registry using a **Personal
+Access Token** stored as the repo secret `REGISTRY_PUSH_PAT` — the built-in per-run `GITEA_TOKEN`
+cannot push to the package registry on this instance.
+
+- **Scope:** `write:package` (`read:package` too if the workflow ever needs to pull the image
+  back before tagging).
+- **Owner:** TODO — fill in once the token is generated (Settings → Applications → Generate New
+  Token on the account that should own the published packages).
+
+## Consuming this image
+
+In `Project4x`'s `.gitea/workflows/export-template.yml`:
+
+```diff
+- container: barichello/godot-ci:4.7.1
++ container: git.arthurerlich.de/haylan/godot-ci:4.7.1
+```