So for the happy path, are you declaring the entire development toolchain on a project by project basis? Any chance you have some recommended resources that show the way you're using it? I was hoping to use it for Haskell but I'd love to see a guide targeted to new users that shows what you're describing.

Yeah, each project has its own default.nix that might look something like this:

    {
      pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/cd63096.tar.gz") { }
    } :
    with pkgs;
    let
      mypackage = callPackage ./mypackage.nix { stdenv = stdenv; python = python3; };
    in
    dockerTools.buildImage {
      contents = [ mypackage ];
    }

You then write a package definition in mypackage.nix, following the template of packaging things in nixpkgs.

(Dev environments work in a similar fashion except with mkShell instead of dockerTools.buildImage.)

I haven't used Nix flakes but I think it develops this simple idea further.