Append /etc/config/lxc-auto with the following:
config container
option name <container>
option timeout <###>
config container
option name <other-container>
option timeout <###>option timeout <###> is the time containers get for graceful shutdowns before being killed (defaults to: 300 seconds).
# (Cron version V5.0 -- $Id: crontab.c,v 1.12 2004/01/23 18:56:42 vixie Exp $)
1 3 * * * fstrim -v /srv >> /srv/log/trim.log 2>&1
30 3 1 * * lxc-backup -a >> /srv/log/lxc-backup.log 2>&1
30 3 15 * * lxc-backup -s -a >> /srv/log/lxc-snapshot.log 2>&1