ci: add GitHub Actions workflow to publish image to GHCR

Mirrors the existing .gitea/workflows/docker-publish.yml but targets
ghcr.io on the GitHub mirror. Simpler than the Gitea version since the
built-in GITHUB_TOKEN has native package-write rights on GHCR, no PAT
secret needed.

.github/workflows/docker-publish.yml

@@ -0,0 +1,37 @@
+name: Publish Docker image
+
+on:
+  push:
+    branches: [main, master]
+    tags: ['v*']
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Compute lowercase image name
+        run: echo "IMAGE_NAME=$(echo '${{ github.repository }}' | tr '[:upper:]' '[:lower:]')" >> "$GITHUB_ENV"
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: |
+            ghcr.io/${{ env.IMAGE_NAME }}:latest
+            ghcr.io/${{ env.IMAGE_NAME }}:${{ github.sha }}