fix(ci): use manifest-descriptor prefix for index-level annotations

Annotations without a prefix land on the per-platform image manifest,
not on the OCI image index. GHCR reads the package description from
the index level, so the description was silently ignored.
The manifest-descriptor: prefix tells buildx to write annotations
to the image index manifest entries instead.

Author: j4n

.github/workflows/docker-ci.yaml

@@ -160,11 +160,13 @@ jobs:
             echo "EOF"
           } >> "$GITHUB_OUTPUT"
 
-          # Index-level annotations so GHCR picks up the description
+          # Index-level annotations so GHCR picks up the description.
+          # manifest-descriptor: targets the OCI image index; without
+          # the prefix annotations only land on the per-platform manifest.
           {
             echo "annotations<<EOF"
-            echo "org.opencontainers.image.title=chatmail-relay"
-            echo "org.opencontainers.image.description=${DESC}"
+            echo "manifest-descriptor:org.opencontainers.image.title=chatmail-relay"
+            echo "manifest-descriptor:org.opencontainers.image.description=${DESC}"
             echo "EOF"
           } >> "$GITHUB_OUTPUT"