code: func (c *Cluster) TagImage(name string, opts docker.TagImageOptions) error {
  	img, err := c.storage().RetrieveImage(name)
  	if err != nil {
  		return err
  	}
  	node, err := c.getNodeByAddr(img.LastNode)
  	if err != nil {
  		return err
  	}
  	err = node.TagImage(name, opts)
  	if err != nil {
  		return wrapError(node, err)
  	}
  	key := imageKey(opts.Repo, opts.Tag)
  	return c.storage().StoreImage(key, img.LastId, node.addr)
  }
docs: // TagImage adds a tag to the given image, returning an error in case of
  // failure.