ci: build and publish Docker image to GHCR

.github/workflows/docker-publish.yml

@@ -0,0 +1,68 @@
+name: Build and Publish Docker Image
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+    tags: ["v*.*.*"]
+
+concurrency:
+  group: docker-publish-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      # Only log in when we're actually going to push. Pull requests (including
+      # those from forks) build the image to validate the Dockerfile but never
+      # push, so they need no registry credentials or secrets.
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Tagging scheme:
+      #   push to main  -> :latest, :sha-<short>
+      #   tag v1.2.3    -> :stable, :1.2.3, :1.2
+      #   pull request  -> (none; build only)
+      # :latest tracks main HEAD; :stable tracks the newest release tag.
+      - name: Compute tags and labels
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          flavor: |
+            latest=false
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,prefix=sha-,enable={{is_default_branch}}
+            type=raw,value=stable,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          # Read the layer cache on every run; only write it when pushing, since
+          # fork pull requests can't write to the Actions cache.
+          cache-from: type=gha
+          cache-to: ${{ github.event_name != 'pull_request' && 'type=gha,mode=max' || '' }}