code: func overlayRender(cfg RunConfig, treeStoreID string, cdir string, dest string, appName string) error {
  	cachedTreePath := cfg.TreeStore.GetRootFS(treeStoreID)
  	mc, err := prepareOverlay(cachedTreePath, treeStoreID, cdir, dest, appName, cfg.MountLabel,
  		cfg.RktGid, common.DefaultRegularDirPerm)
  	if err != nil {
  		return errwrap.Wrap(errors.New("problem preparing overlay directories"), err)
  	}
  	if err = overlay.Mount(mc); err != nil {
  		return errwrap.Wrap(errors.New("problem mounting overlay filesystem"), err)
  	}
  
  	return nil
  }
docs: // overlayRender renders the image that corresponds to the given hash using the
  // overlay filesystem. It mounts an overlay filesystem from the cached tree of
  // the image as rootfs.