Because "only one process in a container" is a dangerous rule (because it has so many exceptions). In certain cases, that idea makes sense, but you shouldn't contort your app such that you only have one process in every container. Not to mention that there are other issues (PID 1 has unique problems that databases and your app aren't used to having to deal with).

maybe you should read this [1]. We have always run all processes under a supervisor.

[1] https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zomb...

I think this was the problem [1] was targeting [1] http://engineeringblog.yelp.com/2016/01/dumb-init-an-init-fo...

One of many.

Which is why u think might as well get systemd working on docker...

I see, so it looks like if your process spawns other processes and doesn't reap them when they die, you end up in trouble with zombie processes in docker.