Docker does not necessarily have to be single-process containers.
See https://phusion.github.io/baseimage-docker/ as an example of a full-OS docker base image designed to be lightweight.
These days, you can run the full OS inside docker[1][2], but you'll likely be running more than you need to.
[1] https://github.com/dotcloud/docker/issues/1024 [2] https://github.com/dotcloud/docker/issues/2276