code: func validateLogOpt(cfg map[string]string) error {
  	for key := range cfg {
  		switch key {
  		case "labels":
  		case "labels-regex":
  		case "env":
  		case "env-regex":
  		case "tag":
  		default:
  			return fmt.Errorf("unknown log opt '%s' for journald log driver", key)
  		}
  	}
  	return nil
  }
docs: // We don't actually accept any options, but we have to supply a callback for
  // the factory to pass the (probably empty) configuration map to.