systemd and systemctl within Ubuntu Docker images

Question

It seems systemd is not active or available in Ubuntu Docker images.

I'm running Docker containers from the ubuntu:16.04 and ubuntu:16.10 images.

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

My main question is: How can systemd and systemctl be activated for use in Ubuntu Docker images?

Why is systemd not active in Ubuntu Docker containers? Is systemd not used in instantiating the container?

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. Is there any documentation giving a full explanation?

Answer 1

This is by design. Docker should be running a process in the foreground in your container and it will be spawned as PID 1 within the container's pid namespace. Docker is designed for process isolation, not for OS virtualization, so there are no other OS processes and daemons running inside the container (like systemd, cron, syslog, etc), only your entrypoint or command you run.

If they included systemd commands, you'd find a lot of things not working since your entrypoint replaces init. Systemd also makes use to cgroups which docker restricts inside of containers since the ability to change cgroups could allow a process to escape the container's isolation. Without systemd running as init inside your container, there's no daemon to process your start and stop commands.