code: func (dc *DriverCall) AppendSpec(spec *volume.Spec, host volume.VolumeHost, extraOptions map[string]string) error {
  	optionsForDriver, err := NewOptionsForDriver(spec, host, extraOptions)
  	if err != nil {
  		return err
  	}
  
  	jsonBytes, err := json.Marshal(optionsForDriver)
  	if err != nil {
  		return fmt.Errorf("Failed to marshal spec, error: %s", err.Error())
  	}
  
  	dc.Append(string(jsonBytes))
  	return nil
  }
docs: // AppendSpec appends volume spec to driver call argument list