Interestingly, systemd maintainers have consistently refused to make available a Docker compatible version of systemd to run as the init 1.
Everyone brings their own here (e.g. http://phusion.github.io/baseimage-docker/) like supervisord, runit,etc.
What would have been ideal is for a docker-compatible systemd to run inside the container.
IMHO the maintainers are bent on creating a competing standard (like this one) and don't want to build anything that brings the advantages of systemd to the Docker ecosystem.
Since version 1.13 Docker has a built-in init based on https://github.com/krallin/tini