code: func NewArchiver(idMapping *idtools.IdentityMapping) *archive.Archiver {
  	if idMapping == nil {
  		idMapping = &idtools.IdentityMapping{}
  	}
  	return &archive.Archiver{
  		Untar:     Untar,
  		IDMapping: idMapping,
  	}
  }
docs: // NewArchiver returns a new Archiver which uses chrootarchive.Untar