docker-run-systest.sh does not work anymore

[grondo]

Starting today, the docker-based "system instance" tests started failing. The issue seems to be related to the version of systemd in the container. I'm unsure if we're seeing the same thing, but this issue at least exhibits similar symptoms:

moby/moby#42275

I was able to reproduce the failure locally, and verify that the basic fluxorma docker container does not work either.

I also verified that podman with --systemd=always works, so perhaps that's a path forward.