You can edit the flake.nix file, and modify basePackages in the devShell section, set env vars in a .env file, and then run nix develop
(the current example bootstraps up a whole posgresql db server and so requires the env vars seen in hooks to be set)
If all env vars are set, then supporting services (just postgres in this case) can be run with overmind start -D which uses the Procfile for local development.
In development shell context, all of the mix deps are installed into gitignored .nix-mix and .nix-hex, so that you can maintain multiple dev envs with unique deps, versions of elixir and postgres etc on one machine.
Once you run nix develop you should also run
(nix:nix-shell-env) sam@dev:~/phoenix_nix_flake_example$ mix archive.install hex phx_new
Resolving Hex dependencies...
Resolution completed in 0.009s
New:
phx_new 1.7.11
* Getting phx_new (Hex package)
All dependencies are up to date
Compiling 11 files (.ex)
Generated phx_new app
Generated archive "phx_new-1.7.11.ez" with MIX_ENV=prod
Are you sure you want to install "phx_new-1.7.11.ez"? [Yn] y
* creating /home/sam/phoenix_nix_flake_example/.nix-mix/archives/phx_new-1.7.11
now you can just run the phoenix as normal in the dev shell with mix -S iex phx.server