docker: GitHub doesn't show labels due to multi-arch image -> add annotation level index

.github/workflows/docker.yml

@@ -48,6 +48,10 @@ jobs:
             type=raw,value=latest,enable={{is_default_branch}}
           labels: |
             org.opencontainers.image.created={{commit_date 'YYYY-MM-DDTHH:mm:ss.SSS[Z]'}}
+        env:
+          # otherwise labels are not shown on GitHub due to multi-arch image: https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#adding-a-description-to-multi-arch-images
+          # https://github.com/docker/metadata-action#annotations
+          DOCKER_METADATA_ANNOTATIONS_LEVELS: manifest,index
       -
         name: Login to Docker Hub
         uses: docker/login-action@v3