I'd be curious to see services designed to run as PID 1 inside containers, and contain or run nothing else other than the required binaries. Maybe someone is doing this.
Pretty sure this is what tini[1] is for, and there's supervisord[2] for more complex use cases, e.g. running multiple processes. Neither are a full replacement for traditional systemd, which I heard you can technically run in a container these days, but I've never seen anyone try.