sync: Add a new flag to enforce syncing only signed images, closes #455

sync: When checking if a image is already synced also check for changes in upstream signatures.

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

pkg/storage/s3/storage.go

@@ -495,6 +495,7 @@ func (is *ObjectStorage) PutImageManifest(repo string, reference string, mediaTy
 			}
 			// manifest contents have changed for the same tag,
 			// so update index.json descriptor
+
 			is.log.Info().
 				Int64("old size", desc.Size).
 				Int64("new size", int64(len(body))).