code: func (u *CopyOnWriteFs) Rename(oldname, newname string) error {
  	b, err := u.isBaseFile(oldname)
  	if err != nil {
  		return err
  	}
  	if b {
  		return syscall.EPERM
  	}
  	return u.layer.Rename(oldname, newname)
  }
docs: // Renaming files present only in the base layer is not permitted