It's not why do you need it, it's that there's 2000 people that know how to do it with docker for every person that knows how to do it with cgroups. And your question is probably just signaling you're in the second group (I'm in the first).

Skill issue ig. The ease of portability and I like Swarm a lot.

Would be curious about any tutorials you recommend on doing that.

Systemd runs things in cgroups by default, so installing the postgres package from your systemd distro's official repos and ensuring that is enabled and running will get you postgres in cgroups to start with. From there you can check the systemd.resource-control(5) man page for some potentially interesting tunables.