Handle the zombie-reaping problem

[smuu]

If the primary process (PID 1) in the container spawns child processes, we need to handle the zombie-reaping problem, as the container runtime is not handling this.

Nice blog article that explains the issue and explains how to fix it:
https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem