Update Docker-Content-Digest if manifest list is rewritten

If the client doesn't support manifest lists, the registry will
rewrite a manifest list into the old format. The Docker-Content-Digest
header should be updated in this case.

Signed-off-by: Oleg Bulatov <oleg@bulatov.me>

registry/handlers/manifests.go

@@ -183,6 +183,8 @@ func (imh *manifestHandler) GetManifest(w http.ResponseWriter, r *http.Request)
 			if err != nil {
 				return
 			}
+		} else {
+			imh.Digest = manifestDigest
 		}
 	}