code: func (i *Image) Metadata(tagName string, v interface{}) error {
  	layerID, ok := i.tags[tagName]
  	if !ok {
  		return fmt.Errorf("can't find tag '%s' for image '%s'", tagName, i.Name)
  	}
  
  	err := i.parseResponse(fmt.Sprintf("v1/images/%s/json", layerID), &v)
  	if err != nil {
  		return err
  	}
  	return nil
  }
docs: // Metadata unmarshals a Docker image metadata into provided 'v' interface.