code: func LookupUID(uid int) (user.User, error) {
  	// first try a local system files lookup using existing capabilities
  	usr, err := user.LookupUid(uid)
  	if err == nil {
  		return usr, nil
  	}
  	// local files lookup failed; attempt to call `getent` to query configured passwd dbs
  	return getentUser(fmt.Sprintf("%s %d", "passwd", uid))
  }
docs: // LookupUID uses traditional local system files lookup (from libcontainer/user) on a uid,
  // followed by a call to `getent` for supporting host configured non-files passwd and group dbs