Indeed you can skip most of the unnecessary additional mount requirements with systemd-nspawn. If /dev/sde1 (your root partition) is mounted to /tmp/rescue just

Indeed you can skip most of the unnecessary additional mount requirements with systemd-nspawn. If /dev/sde1 (your root partition) is mounted to /tmp/rescue just run: systemd-nspawn --directory /tmp/rescue --boot -- --unit rescue.target It should automatically find the boot partition and mount it as well.