A single container for everything gets away from the point of containerization. If you have a single container for lemmy-ui, lemmy backend, and postgres, you need to rebuild that container whenever any one of those applications gets an update, and they could start to interfere with each other. Keeping them in separate containers makes everything a lot cleaner, it just requires something like docker compose to put it all together.
Did you try the Ansible install? Provided you're installing onto a supported Debian/Ubuntu version, I found it fairly straightforward.
That's the fun part, now they have to actually find folks to effectively moderate that sub.