code: func (a *Driver) createDirsFor(id string) error {
  	paths := []string{
  		"mnt",
  		"diff",
  	}
  
  	rootUID, rootGID, err := idtools.GetRootUIDGID(a.uidMaps, a.gidMaps)
  	if err != nil {
  		return err
  	}
  	// Directory permission is 0755.
  	// The path of directories are <aufs_root_path>/mnt/<image_id>
  	// and <aufs_root_path>/diff/<image_id>
  	for _, p := range paths {
  		if err := idtools.MkdirAllAndChown(path.Join(a.rootPath(), p, id), 0755, idtools.Identity{UID: rootUID, GID: rootGID}); err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // createDirsFor creates two directories for the given id.
  // mnt and diff