code: func (i *Image) TagLayerID(tagName string) (string, error) {
  	layerID, ok := i.tags[tagName]
  	if !ok {
  		return "", fmt.Errorf("can't find tag '%s' for image '%s'", tagName, i.Name)
  	}
  
  	return layerID, nil
  }
docs: // TagLayerID returns a layer ID for a given tag.