GitHub: specify docker image platforms

To enable building docker images for ARM64 platforms as well,
we just need to specify the desired target platforms and the Docker
Buildx service will do the job for us (provided the base images support
the given platforms, which is the case for golang).

.github/workflows/docker.yml

@@ -37,6 +37,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           push: true
+          platforms: linux/amd64,linux/arm64
           tags: "${{ env.DOCKER_REPO }}/${{ env.DOCKER_IMAGE }}:${{ env.RELEASE_VERSION }}"
           build-args: checkout=${{ env.RELEASE_VERSION }}