Use https://lemmy.ml/c/nixos instead.
i just got it running today, behind nginx. it did not work out of the box: i had to patch the upstream database migration scripts otherwise it would crash on start with some weird Postgres permissions edgecase. after that it's fine: idling at 200 MB and trivial CPU.
i'll try and upstream stuff to make it easier but until then here's what works for me (you can also find this by searching lemmy in a NUR checkout):
Care to upstream those?
i threw up a PR. take a look, give it a whirl if you like: https://github.com/NixOS/nixpkgs/pull/232536
even with this upstream PR i deploy it with this two-line override. i'm suspect of a bug in upstream lemmy such that it expects to have superuser database access during the initial migrations -- which that local override patches. it'd be super helpful to get confirmation from other users if this that patch is also in need of upstreaming or if it's more due to my specific postgres configuration.