Add GitHub Actions workflow to build and publish image to GHCR

Builds the production Docker image and pushes it to
ghcr.io/dkam/clinch on pushes to main (edge + sha tags) and on v*
release tags (vX.Y.Z, vX.Y, latest). amd64 only, with GHA layer caching.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/build.yml

@@ -0,0 +1,56 @@
+name: Build and publish image
+
+on:
+  push:
+    branches: [ main ]
+    tags: [ 'v*' ]
+
+# Only one build per ref at a time; cancel superseded main builds.
+concurrency:
+  group: build-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref == 'refs/heads/main' }}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write  # Required to push to GHCR
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract image metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=edge,branch=main
+            type=sha,prefix=sha-,format=short,enable={{is_default_branch}}
+            type=semver,pattern=v{{version}}
+            type=semver,pattern=v{{major}}.{{minor}}
+          flavor: |
+            latest=auto
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max