ci: Document default labels and annotations

.github/workflows/release-image.yml

@@ -55,6 +55,7 @@ jobs:
           images: |
             docker.io/${{ env.IMAGE_NAME }}
             ghcr.io/${{ env.IMAGE_NAME }}
+          # default labels & annotations: https://github.com/docker/metadata-action/blob/master/src/meta.ts#L509
 
       # These steps work around docker mount caches not being cached between runs in CI providers.
       # It manually injects the mounts into Docker.