code: func (r *RootFS) ChainID() layer.ChainID {
  	if runtime.GOOS == "windows" && r.Type == typeLayersWithBase {
  		logrus.Warnf("Layer type is unsupported on this platform. DiffIDs: '%v'", r.DiffIDs)
  		return ""
  	}
  	return layer.CreateChainID(r.DiffIDs)
  }
docs: // ChainID returns the ChainID for the top layer in RootFS.