In the beginning, Docker DID have "standalone mode" where it would launch just one container as a child process. That was actually an easier way to manage the mounts and cgroups necessary to stand up a container. I made a ticket to bring it back after they removed it, and it was closed with a wontfix. The cynic in me says it was done more for commercial reasons (they wanted a more full featured daemon on the server doing things they could charge for) as opposed to just being a little shim that just did one thing.