Simplest derivation ever

flake.lock

{
  "nodes": {
    "flake-utils": {
      "inputs": {
        "systems": "systems"
      },
      "locked": {
        "lastModified": 1689068808,
        "narHash": "sha256-6ixXo3wt24N/melDWjq70UuHQLxGV8jZvooRanIHXw0=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "919d646de7be200f3bf08cb76ae1f09402b6f9b4",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1691186842,
        "narHash": "sha256-wxBVCvZUwq+XS4N4t9NqsHV4E64cPVqQ2fdDISpjcw0=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "18036c0be90f4e308ae3ebcab0e14aae0336fe42",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixos-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": "nixpkgs"
      }
    },
    "systems": {
      "locked": {
        "lastModified": 1681028828,
        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
        "owner": "nix-systems",
        "repo": "default",
        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
        "type": "github"
      },
      "original": {
        "owner": "nix-systems",
        "repo": "default",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  description = "mdeverything";

  nixConfig.extra-trusted-substituters = "http:/localhost:8080";

  nixConfig.extra-trusted-public-keys =
    "ci:ownPA2gWz4niO8YrdaJOaFQVdkA9SeqZ45g551NDQS0=";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = inputs@{ self, nixpkgs, flake-utils, ... }:
    # flake-utils.lib.eachDefaultSystem
    flake-utils.lib.eachSystem [ flake-utils.lib.system.x86_64-linux ]
      (system:
        let
          pkgs = nixpkgs.legacyPackages.${system};
        in
        rec
        {
          packages.mdeverything =
            pkgs.stdenv.mkDerivation
              {
                name = "mdeverything";
                src = ./.;
                buildInputs = [ pkgs.coreutils ];

                unpackPhase = "true";

                buildPhase = ''
                  cd $src
                  md5sum in.txt > "$out"
                '';
              };

          defaultPackage = packages.mdeverything;

          devShell =
            pkgs.mkShell {
              buildInputs = [
              ];
              inputsFrom = [
                # self.defaultPackage.${system}.env
              ];
            };
        });
}

in.txt

1