ci: publish multi-arch container image to GHCR

Build linux/amd64 + linux/arm64 from the existing Dockerfile on every push
to main, on git tags v*.*.*, and on manual workflow_dispatch. Pushes to
ghcr.io/metalfinger/refboard with tags :latest (default branch), :sha-XXX
(every push), and semver tags on releases. Uses GHA cache for speed.

Tightens .dockerignore so multi-arch builds don't ship docs/, examples/,
scripts/, .github/, or .docker-data into the image context.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.dockerignore

@@ -1,3 +1,10 @@
 **/node_modules
 **/.git
 **/.env
+.github
+.docker-data
+docs
+examples
+scripts
+*.md
+!README.md

.github/workflows/publish-image.yml

@@ -0,0 +1,54 @@
+name: Publish container image
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*.*.*']
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-qemu-action@v3
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=sha,prefix=sha-,format=short
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max