I havent changed anything inside the container as far as i’m aware, so i’m not personally using either netplan or interfaces. 
here is the output on the container:
root@test:~# cat /etc/network/interfaces
# ifupdown has been replaced by netplan(5) on this system. See
# /etc/netplan for current configuration.
# To re-enable ifupdown on this system, you can run:
# sudo apt install ifupdown
root@test:~# cat /etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource. Changes
# to it will not persist across an instance reboot. To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
version: 2
ethernets:
eth0:
dhcp4: true
and the output of ps aux:
joey@joey-ThinkPad-T460:~/projects/OPC_UA$ lxc launch ubuntu: test2
Creating test2
Starting test2
(venv) joey@joey-ThinkPad-T460:~/projects/OPC_UA$ lxc exec test2 ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 77560 5764 ? Ss 10:03 0:00 /sbin/init
root 85 0.0 0.0 78492 7228 ? Ss 10:03 0:00 /lib/systemd/systemd-journald
root 93 0.0 0.0 33348 2052 ? Ss 10:03 0:00 /lib/systemd/systemd-udevd
systemd+ 205 0.0 0.0 71856 3780 ? Ss 10:03 0:00 /lib/systemd/systemd-networkd
root 206 0.0 0.0 61828 2012 ? Ss 10:03 0:00 /lib/systemd/systemd-networkd-wait-online
systemd+ 207 0.0 0.0 70636 3976 ? Ss 10:03 0:00 /lib/systemd/systemd-resolved
root 208 0.0 0.0 37792 1972 ? Rs+ 10:03 0:00 ps aux