build: publish multi-platform image (amd64, arm64, arm/v7)

The builder stage runs on the build platform and cross-compiles Rust,
with GCC cross compilers for bundled SQLite, so only the small runtime
stage runs under QEMU. The workflow sets up QEMU via the Docker Hub
mirror and pushes all three platforms under the same tags.

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

.github/workflows/docker-publish.yml

@@ -55,6 +55,13 @@ jobs:
           echo "image=$image" >> "$GITHUB_OUTPUT"
           echo "Publishing $image:$tag as $REGISTRY_USER"
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+        with:
+          # Only the runtime stage runs emulated; Rust is cross-compiled natively.
+          image: ${{ vars.DOCKERHUB_MIRROR || 'mirror.gcr.io' }}/tonistiigi/binfmt:latest
+          platforms: arm64,arm
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
         with:
@@ -75,6 +82,7 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
           push: true
           tags: |
             ${{ steps.meta.outputs.image }}:${{ steps.meta.outputs.version_tagname }}