code: func validateAttach(val string) (string, error) {
  	s := strings.ToLower(val)
  	for _, str := range []string{"stdin", "stdout", "stderr"} {
  		if s == str {
  			return s, nil
  		}
  	}
  	return val, errors.Errorf("valid streams are STDIN, STDOUT and STDERR")
  }
docs: // validateAttach validates that the specified string is a valid attach option.