To run systemd inside Docker containers easily, I suggest using Docker with Sysbox (a new type of runc):
$ docker run --runtime=sysbox-runc -it nestybox/ubuntu-bionic-systemd
Sysbox sets up the container such that it can run systemd (and even Docker & K8s) with full isolation from the underlying host (via the Linux user-namespace). It’s meant to enable Docker to deploy “VM-like” containers (similar to LXD).
Hope this helps!