code: func mountOverlay(pod *pkgPod.Pod, app *schema.RuntimeApp, dest string) error {
  	if _, err := os.Stat(dest); err != nil {
  		return err
  	}
  
  	s, err := imagestore.NewStore(getDataDir())
  	if err != nil {
  		return errwrap.Wrap(errors.New("cannot open store"), err)
  	}
  
  	ts, err := treestore.NewStore(treeStoreDir(), s)
  	if err != nil {
  		return errwrap.Wrap(errors.New("cannot open treestore"), err)
  	}
  
  	treeStoreID, err := pod.GetAppTreeStoreID(app.Name)
  	if err != nil {
  		return err
  	}
  	lower := ts.GetRootFS(treeStoreID)
  	imgDir := filepath.Join(filepath.Join(pod.Path(), "overlay"), treeStoreID)
  	if _, err := os.Stat(imgDir); err != nil {
  		return err
  	}
  	upper := filepath.Join(imgDir, "upper", app.Name.String())
  	if _, err := os.Stat(upper); err != nil {
  		return err
  	}
  	work := filepath.Join(imgDir, "work", app.Name.String())
  	if _, err := os.Stat(work); err != nil {
  		return err
  	}
  
  	if err := overlay.Mount(&overlay.MountCfg{lower, upper, work, dest, ""}); err != nil {
  		return errwrap.Wrap(errors.New("problem mounting overlayfs directory"), err)
  	}
  
  	return nil
  }
docs: // mountOverlay mounts the app from the overlay-rendered pod to the destination directory.