cat /var/lib/lxc/nameofthecontain/tmp_root_pass
>>> temporary password
or
chroot var/lib/lxc/nameofthecontain/rootfs
passwd >>>> then enter a new password
cat /var/lib/lxc/nameofthecontain/tmp_root_pass
>>> temporary password
or
chroot var/lib/lxc/nameofthecontain/rootfs
passwd >>>> then enter a new password