Baseimage-docker uses runit exactly to not run a full distribution in the container. Upstart tries to boot a full Ubuntu. A full Ubuntu is not necessary inside

Baseimage-docker uses runit exactly to not run a full distribution in the container. Upstart tries to boot a full Ubuntu. A full Ubuntu is not necessary inside the container. Therefore, baseimage-docker provides a custom init system that boots only the minimal subset of Ubuntu that is necessary for it to run correctly in Docker.