shared/idmap/idmapset_linux.go
func (e *IdmapEntry) shift_into_ns(id int64) (int64, error) {
	if id < e.Nsid || id >= e.Nsid+e.Maprange {
		// this mapping doesn't apply
		return 0, fmt.Errorf("ID mapping doesn't apply")
	}

	return id - e.Nsid + e.Hostid, nil
}
func (e *IdmapEntry) shift_into_ns(id int64) (int64, error) {
	if id < e.Nsid || id >= e.Nsid+e.Maprange {
		// this mapping doesn't apply
		return 0, fmt.Errorf("ID mapping doesn't apply")
	}

	return id - e.Nsid + e.Hostid, nil
}
/*
 * Shift a uid from the host into the container
 * I.e. 0 -> 1000 -> 101000
 */