ci: set docker annotation level to index

https://github.com/astral-sh/uv/blob/0.9.13/.github/workflows/build-docker.yml#L279-L281

Author: thegamecracks

.github/workflows/docker.yml

@@ -43,6 +43,9 @@ jobs:
 
       - uses: docker/metadata-action@v5.10.0
         id: meta
+        # ghcr.io prefers index level annotations
+        env:
+          DOCKER_METADATA_ANNOTATIONS_LEVELS: index
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           tags: |