GitHub: specify docker image platforms

Author: guggero

.github/workflows/docker.yml

@@ -38,6 +38,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           push: true
+          platforms: linux/amd64,linux/arm/v7,linux/arm64
           tags: "${{ env.DOCKER_REPO }}/${{ env.DOCKER_IMAGE }}:${{ env.RELEASE_VERSION }}"
           build-args: checkout=${{ env.RELEASE_VERSION }}