code: func (c *Cluster) PushImage(opts docker.PushImageOptions, auth docker.AuthConfiguration) error {
  	key := imageKey(opts.Name, opts.Tag)
  	img, err := c.storage().RetrieveImage(key)
  	if err != nil {
  		return err
  	}
  	node, err := c.getNodeByAddr(img.LastNode)
  	if err != nil {
  		return err
  	}
  	node.setPersistentClient()
  	return wrapError(node, node.PushImage(opts, auth))
  }
docs: // PushImage pushes an image to a remote registry server, returning an error in
  // case of failure.