feat: add arm64 Docker image support (closes #44)

Add QEMU and multi-platform build (linux/amd64 + linux/arm64) to
GitHub Actions workflow, enabling self-hosting on Raspberry Pi and
other ARM64 devices.

.github/workflows/docker-publish.yml

@@ -40,6 +40,9 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
             type=sha
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -47,6 +50,7 @@ jobs:
         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 }}