ci: publish a container image to ghcr.io on each release

Builds the image and pushes it to ghcr.io/uio-gaming/bebbes, tagged with the release version and latest. It runs as a job in the release-please workflow, because releases created with GITHUB_TOKEN don't trigger other workflows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/release-please.yml

@@ -12,8 +12,51 @@ permissions:
 jobs:
   release-please:
     runs-on: ubuntu-latest
+    outputs:
+      release_created: ${{ steps.release.outputs.release_created }}
+      version: ${{ steps.release.outputs.version }}
     steps:
       - uses: googleapis/release-please-action@v4
+        id: release
         with:
           config-file: release-please-config.json
           manifest-file: .release-please-manifest.json
+
+  # Releases made by release-please don't trigger other workflows (GitHub ignores events
+  # created with GITHUB_TOKEN), so the image is published from here instead.
+  publish-image:
+    needs: release-please
+    if: needs.release-please.outputs.release_created == 'true'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v5
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/metadata-action@v5
+        id: meta
+        with:
+          images: ghcr.io/uio-gaming/bebbes
+          tags: |
+            type=semver,pattern={{version}},value=${{ needs.release-please.outputs.version }}
+            type=semver,pattern={{major}}.{{minor}},value=${{ needs.release-please.outputs.version }}
+            type=semver,pattern={{major}},value=${{ needs.release-please.outputs.version }}
+            type=raw,value=latest
+
+      - uses: docker/setup-buildx-action@v3
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max