Use gdraheim/docker-systemctl-replacement in all baseimages

[tknerr]

I have made quite positive experience with https://github.com/gdraheim/docker-systemctl-replacement so far, and it totally fits the idea of the vagrant-docker-baseimages.

We should add it to all systemd based baseimages in that repo.

• at least, so that we have a "docker-compatible" systemctl command available
• I failed replacing the current sshd entrypoint with systemctl as the entrypoint, I could get it working, but sshd started via systemd regularly crashed and I couldn't figure out why -- so that is not working yet and thus the current sshd entrypoint should be kept in the first step
• for 16.04 and 18.04 use systemctl.py, for 20.04 use systemctl3.py
• consider pinning to a specific commit so we get stable versions

Let's do that only after #17 has been completed, i.e. with versioning in place so that we still have the older containers (with original systemd) published and available, if someone still needs them