I am so outdated. I not designed system start/stop boot scripts since the "#_something" days.
I have an app based on docker compose that I need to start at boot and stop before shutdown.
what is now considered the proper way to do that: sysctl, systemctl, service, or something else.
since this is dependent on docker running which depends on other things. I am thinking systemctl
What is the current proper way to start software at boot? Can you recommend good documentation and examples to that recommendation?
My actual usage is RHEL 7.6+ and Unbuntu 18.4+ but I would like to support the broadest base of linux/unix platforms
RHEL 7 and Ubuntu 18.04 both use systemd, as do essentially all major distros now. You'll need to write a systemd unit file for your software. You mentioned the "service" command, that's a Red Hat legacy thing and, in almost all cases, it simply redirects to systemctl (systemd) nowadays. You also mentioned "sysctl" but that adjusts kernel parameters and has nothing to do with this topic.
Red Hat has good documentation on systemd, including creating unit files, here's the link for RHEL 7:
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/chap-managing_services_with_systemd
You could also go straight to the source, although it's nowhere near as well-organized as Red Hat's documentation IMO:
https://www.freedesktop.org/wiki/Software/systemd/