With --shell, pass --read-only through to systemd-nspawn
With --shell, pass --read-only through to systemd-nspawn

It seems systemd-nspawn has a bug where --read-only doesn't actually
work, but this should not hurt in either case.

Edit: https://github.com/systemd/systemd/pull/10481 for systemd-nspawn.

UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi,UbiCastTeam/mkosi