https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html
chroot improved.
on arch - i needed qemu-user-static AUR. it takes FOREVER to biuld.
the conf files were absent and so systemd-binfmt (instead of binfmt-support) was failing to load. followed this and resolved: https://github.com/computermouth/qemu-static-conf/blob/master/README.md
create loopback of the raspberrypi.img file
sudo losetup -P -f --show some/path/to/raspbian.img
assuming loop0, mount the boot and rootfs
sudo mkdir-P /mnt/rpi