emacs-with-config

This function will give you an Emacs derivation which loads the given script alongside with all the packages mentioned in it (doom-themes, undo-tree and magit in this case):

let
pkgs = import <nixpkgs> {};
emacsWithConfig = pkgs.callPackage 
  (builtins.fetchTarball "https://github.com/utdemir/emacs-with-config/archive/master.tar.gz")
  {};
in
emacsWithConfig ''
  (require 'use-package)

  (use-package doom-themes
    :config
    (load-theme 'doom-molokai t))

  (use-package undo-tree
    :defer 2
    :init
    (setq undo-tree-visualizer-timestamps t)
    :config
    (global-undo-tree-mode))

  (use-package magit
    :bind
    ("C-x m" . magit-status))
''

To be more precise, it will:

1. Parse the given script to find the use-package usages and collect the required packages.
2. Use pkgs.emacsWithPackages to get an emacs distribution with the required packages.
3. Bytecompile the given script.
4. Wrap pkgs.emacs to load the given script by default.