I would disagree that containers aren't supposed to run more than one process. It's just discouraged because a lot of people aren't well versed in the pitfalls of being PID 1. Fedora's toolbox is a great counter-example, as is systemd now being able to boot up as your PID 1 in some container distros without much modification.
To be fair, even for running a single process the pitfalls are real. I've been seeing Tini[1] a lot for these situations.
I just read in the README that Tini is included by Docker since 1.13 if using --init flag.