Update documentation on the nix build pack
MODIFY
docs/source/config_files.rst
docs/source/config_files.rst
@@ -164,6 +164,27 @@ used for installing libraries.
To see an example R repository, visit our `R
example in binder-examples <https://github.com/binder-examples/r/blob/master/runtime.txt>`_.
+.. _default.nix:
+
+``default.nix`` - the nix package manager
+=========================================
+
+Specify packages to be installed by the `nix package manager <https://github.com/NixOS/nixpkgs>`_.
+When you use this config file all other configuration files (like ``requirements.txt``)
+that specify packages are ignored. When using ``nix`` you have to specify all
+packages and dependencies explicitly, including the Jupyter notebook package that
+repo2docker expects to be installed. If you do not install Jupyter explicitly
+repo2docker will no be able to start your container.
+
+`nix-shell <https://nixos.org/nix/manual/#sec-nix-shell>`_ is used to evaluate
+a ``nix`` expression written in a ``default.nix`` file. Make sure to
+`pin your nixpkgs <https://discourse.nixos.org/t/nixops-pinning-nixpkgs/734>`_
+to produce a reproducible environment.
+
+To see an example repository visit
+`nix binder example <https://gitlab.com/costrouc/nix-binder-example>`_.
+
+
``Dockerfile`` - Advanced environments
======================================
@@ -178,26 +199,3 @@ With Dockerfiles, a regular Docker build will be performed.
See the `Advanced Binder Documentation <https://mybinder.readthedocs.io/en/latest/tutorials/dockerfile.html>`_ for
best-practices with Dockerfiles.
-
-.. _default.nix:
-
-``default.nix``
-~~~~~~~~~~~~~~~
-
-This allows you to use the `nix package manager <https://github.com/NixOS/nixpkgs>`_. It is hard to explain what nix
-is to new users and why it is usefull. If you are inclined please read
-more at the `nix homepage <https://nixos.org/nix/>`_. It is currently
-the largest package repository, offers reproducible builds, multiple
-versions of same package coexisting, source and binary based, and
-packages many languages such as python, R, go, javascript, haskell,
-ruby, etc. .
-
-A ``default.nix`` file allows you to use `nix-shell <https://nixos.org/nix/manual/#sec-nix-shell>`_
-to evaluate a ``nix`` expression to define a reproducible nix environment.
-The only requirement is that you expose a ``jupyter`` command within the shell
-(since jupyterlab is currently what ``repo2docker`` is designed
-around). While the ``nix`` environment does have ``NIX_PATH`` set with
-``nixpkgs=...`` you should not rely on it and make sure to
-`pin your nixpkgs <https://discourse.nixos.org/t/nixops-pinning-nixpkgs/734>`_.
-By doing this you are truley producing a reproducible environment. To see an
-example repository visit a `nix binder example <https://gitlab.com/costrouc/nix-binder-example>`_.

Update documentation on the nix build pack