criu can't seem to dump systemd-logind used in Ubuntu due to what appears to be
https://github.com/checkpoint-restore/criu/issues/1430.
Let's switch to busybox where all the processes hopefully can be dumped.

Closes https://github.com/lxc/lxc/issues/3792Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>