In this case the device is a USB device posing as a serial port.
ls -l /dev/bus/usb/004/005
crw-rw-r-- 1 root root 189, 388 Oct 25 16:19 /dev/bus/usb/004/005
Edit the LXC Config file /var/lib/lxc/<name>
lxc.cgroup.devices.allow = c 189:388 rwm
lxc.mount.entry = /dev/bus/usb/004/005 dev/bus/usb/004/005 none bind,optional,create=file
ls -l /dev/ttyACM0
crwxrwxrwx 1 root dialout 166, 0 Oct 30 13:38 /dev/ttyACM0
Edit the LXC Config file
lxc.cgroup.devices.allow: c 166:* rwm
lxc.mount.entry: /dev/ttyACM0 dev/ttyACM0 none bind,optional,create=file