The thing is, I don't even really like Docker as an init daemon. I have my gripes about Systemd but I see no downsides to not having a long-running daemon for a container engine. Really, whether you need root or not isn't even the most important issue; you can do sudo or suid or whatever with any container engine; Docker just has it be an implicit, unintuitive behavior.

I used to use systemd+rkt for simple container setups when I didn't need all of Kubernetes. Never noticed any downsides versus using Docker, but on the flip side, I had far fewer issues with my containers not properly starting at boot.

I read an article (can't find it now) that said from a previous project the Docker authors concluded they wanted a daemon so they didn't have to do things like file locks, etc. around image management.

Don't know if that accounts for the whole reason or not.

Sounds like it might've been this article?

https://jpetazzo.github.io/2017/02/24/from-dotcloud-to-docke...?

Oh yeah, I read that - surrounding the original authors at dotCloud about their experiences trying to do it in multiple processes IIRC. That said, it seems the problem may be somewhat solved at this point.

The reason boiled down to "it's easier to write a daemon to handle it all". But it's definitely not better than the alternative, nor is the alternative impossible.