Unwrap URL errors on retry

When authorization errors are returned by the token process the error will be wrapped in url.Error.
In order to check the underlying error for retry this error message should be unwrapped.
Unwrapping this error allows failure to push due to an unauthorized response to keep from retrying, possibly resulting in later 429 responses.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

distribution/registry.go

@@ -145,6 +145,8 @@ func retryOnError(err error) error {
 		case errcode.ErrorCodeUnauthorized, errcode.ErrorCodeUnsupported, errcode.ErrorCodeDenied:
 			return xfer.DoNotRetry{Err: err}
 		}
+	case *url.Error:
+		return retryOnError(v.Err)
 	case *client.UnexpectedHTTPResponseError:
 		return xfer.DoNotRetry{Err: err}
 	}