code: func getUserFromImageUser(imageUser string) (*int64, string) {
  	user := parseUserFromImageUser(imageUser)
  	// return both nil if user is not specified in the image.
  	if user == "" {
  		return nil, ""
  	}
  	// user could be either uid or user name. Try to interpret as numeric uid.
  	uid, err := strconv.ParseInt(user, 10, 64)
  	if err != nil {
  		// If user is non numeric, assume it's user name.
  		return nil, user
  	}
  	// If user is a numeric uid.
  	return &uid, ""
  }
docs: // getUserFromImageUser gets uid or user name of the image user.
  // If user is numeric, it will be treated as uid; or else, it is treated as user name.