How can systemd and systemctl be enabled and used in Ubuntu Docker containers?

**Problem** How can `systemd` and `systemctl` be enabled and used in Ubuntu Docker containers? Why isn't systemd enabled by default in these containers, and why is it not considered a best practice in Docker? **Setup** I'm running Docker containers from the ubuntu:16.04 and ubuntu:16.10 images. **Tests** If I execute: `systemctl status ssh` in the 16,04 container the result is `the error Failed to connect to bus: No such file or directory` In the 16.10 container the error is: `bash: systemctl: command not found.` If I do `which systemctl` systemctl is found in the 16.04 container but not in the 16.10 container. I have spotted that `/lib/systemd` exists. I have tried installing `systemd` with: apt-get install systemd libpam-systemd systemd-ui Then which `systemctl` finds `systemctl` in 16.10 but `systemctl status ssh` still gives the error `Failed to connect to bus: No such file or directory` **Questions** Can someone provide a solution for enabling and using systemd in Ubuntu Docker containers? I have failed to find any documentation on this topic for Ubuntu / Ubuntu Docker images, only information on the Ubuntu transition from Upstart to systemd.