An Ubuntu 18.04 Docker image including
$ docker pull roryrjb/ubuntu-systemd:18.04
The official Docker image for 18.04 doesn't include
systemd, this image
does, it's as simple as that.
To run this properly, you'll need to do something like this:
$ docker run \ --tmpfs /run \ -v /sys/fs/cgroup:/sys/fs/cgroup:ro \ --stop-signal SIGRTMIN+3 \ roryrjb/ubuntu-systemd:18.04
See this article for more details on the specifics.