docker container users/groups: can you actually create and use them in heroku?

Question

I know the official Heroku guidance is "don't use privileged users/groups in docker containers", but what I wonder is, can you even create and use your OWN users and groups?? And if so, how?! When I run an "adduser" or "addgroup" in my Dockerfile, I find it seems to have been wiped out when I log into the console of that container deployed to Heroku! Is there some magic I am missing here? Am I supposed to be using a USER statement in my Dockerfile or something? (I wound up resorting to using the "dyno" user that Heroku apparently autogenerates inside my container to run my entrypoint application, but that is highly inconvenient.)