ci: Add multi-arch Docker build/publish and tagged release workflow

Build and push linux/amd64 + linux/arm64 images to GHCR on every push
to master and on v* tags. On v* tags, also generate a changelog from
conventional commits and create a GitHub release.

.github/workflows/docker-publish.yml

@@ -0,0 +1,88 @@
+name: Docker build & release
+
+on:
+  push:
+    branches: [master]
+    tags: ["v*"]
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        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,format=short
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
+
+      - 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: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+  release:
+    needs: build-and-push
+    if: startsWith(github.ref, 'refs/tags/v')
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Generate changelog
+        id: changelog
+        run: |
+          prev_tag=$(git describe --tags --abbrev=0 "${{ github.ref_name }}^" 2>/dev/null || true)
+          if [ -n "$prev_tag" ]; then
+            range="${prev_tag}..${{ github.ref_name }}"
+          else
+            range=""
+          fi
+          .github/generate_changelog.sh "$range" > /tmp/changelog.md
+          cat /tmp/changelog.md
+
+      - name: Create GitHub release
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh release create "${{ github.ref_name }}" \
+            --title "${{ github.ref_name }}" \
+            --notes-file /tmp/changelog.md