code: func (i *ImageService) GetLayerMountID(cid string, os string) (string, error) {
  	return i.layerStores[os].GetMountID(cid)
  }
docs: // GetLayerMountID returns the mount ID for a layer
  // called from daemon.go Daemon.Shutdown(), and Daemon.Cleanup() (cleanup is actually continerCleanup)
  // TODO: needs to be refactored to Unmount (see callers), or removed and replaced
  // with GetLayerByID