Docker: use buildx to build amd64 and arm64 images

.github/workflows/docker.yml

@@ -15,6 +15,10 @@ jobs:
       -
         name: Check out
         uses: actions/checkout@v3
+      -
+        name: Set up QEMU
+        if: github.event_name == 'push'
+        uses: docker/setup-qemu-action@v1
       -
         name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
@@ -78,6 +82,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}