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>

.gitea/workflows/publish.yml

@@ -0,0 +1,21 @@
+name: Publish godot-ci image
+
+on:
+  push:
+    branches: [main]
+    paths: [Dockerfile]
+  workflow_dispatch:
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to Gitea registry
+        run: echo "${{ secrets.REGISTRY_PUSH_PAT }}" | docker login git.arthurerlich.de -u haylan --password-stdin
+
+      - name: Build and push
+        run: |
+          docker build -t git.arthurerlich.de/haylan/godot-ci:4.7.1 .
+          docker push git.arthurerlich.de/haylan/godot-ci:4.7.1

Dockerfile

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+FROM barichello/godot-ci:4.7.1
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libfontconfig1 \
+    build-essential scons pkg-config xz-utils curl \
+    libx11-dev libxcursor-dev libxinerama-dev libgl1-mesa-dev \
+    libasound2-dev libpulse-dev libudev-dev libxi-dev libxrandr-dev libwayland-dev \
+    mingw-w64 \
+    blender \
+  && update-alternatives --set x86_64-w64-mingw32-gcc /usr/bin/x86_64-w64-mingw32-gcc-posix \
+  && update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix \
+  && rm -rf /var/lib/apt/lists/*

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
+```