code: func ValidateLogOpt(cfg map[string]string) error {
  	for key := range cfg {
  		switch key {
  		case "max-file":
  		case "max-size":
  		case "compress":
  		case "labels":
  		case "labels-regex":
  		case "env":
  		case "env-regex":
  		case "tag":
  		default:
  			return fmt.Errorf("unknown log opt '%s' for json-file log driver", key)
  		}
  	}
  	return nil
  }
docs: // ValidateLogOpt looks for json specific log options max-file & max-size.