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