I'm curious what makes you think "multiple instances" when you see this? You realise operating systems have been able to run multiple processes for decades, I assume? And you probably think nothing of running multiple shells, web browsers etc? None of this has anything to do with Docker.

I've noticed a lot of people seem to think Docker is some dark art technology when it's really just an amalgamation of various things that you can do anyway.

Incidentally, when I was on dial up and before I had a home network (just one family PC) I started to learn networking by running things like IRC servers (unrealircd to be specific) and multiple clients locally (including eggies). I was really talking to myself in every sense. Was quite fun to give myself ops etc.