Add FAQ entry and Howto on exporting conda envs

docs/source/faq.rst

@@ -55,6 +55,34 @@ R
 Only R 3.4.4 is currently supported, which is installed via ``apt`` from the
 `ubuntu bionic repository <https://packages.ubuntu.com/bionic/r-base>`_.
 
+
+Why is my repository is failing to build with ``ResolvePackageNotFound`` ?
+--------------------------------------------------------------------------
+
+If you used ``conda env export`` to generate your ``environment.yml`` it will
+generate a list of packages and versions of packages that is pinned to platform
+specific versions. These very specific versions are not available in the linux
+docker image used by ``repo2docker``. A typical error message will look like
+the following::
+
+  Step 39/44 : RUN conda env update -n root -f "environment.yml" && conda clean -tipsy && conda list -n root
+  ---> Running in ebe9a67762e4
+  Solving environment: ...working... failed
+
+  ResolvePackageNotFound:
+  - jsonschema==2.6.0=py36hb385e00_0
+  - libedit==3.1.20181209=hb402a30_0
+  - tornado==5.1.1=py36h1de35cc_0
+  ...
+
+We recommend to use ``conda env export --no-builds -f environment.yml`` to export
+your environment and then edit the file by hand to remove platform specific
+packages like ``appnope``.
+
+See :ref:`export-environment` for a recipe on how to create strict exports of
+your environment that will work with ``repo2docker``.
+
+
 Can I add executable files to the user's PATH?
 ----------------------------------------------
 
@@ -98,7 +126,7 @@ Yes: use the ``--editable`` or ``-E`` flag (don't confuse this with
 the ``-e`` flag for environment variables), and run repo2docker on a
 local repository::
 
-  repo2docker -E my-repository/.
+  repo2docker -E my-repository/
 
 This builds a Docker container from the files in that repository
 (using, for example, a ``requirements.txt`` or ``install.R`` file),

docs/source/howto/export_environment.rst

@@ -0,0 +1,50 @@
+.. _export-environment:
+
+=============================================================================
+How to automatically create a ``environment.yml`` that works with repo2docker
+=============================================================================
+
+This how-to explains how to create a ``environment.yml`` that specifies all
+installed packages and their precise versions from your environment.
+
+
+The challenge
+=============
+
+``conda env export -f environment.yml`` creates a strict export of all packages.
+This is the most robust for reproducibility, but it does bake in potential
+platform-specific packages, so you can only use an exported environment on the
+same platform.
+
+``repo2docker`` uses a linux based image as the starting point for every docker
+image it creates. However a lot of people use OSX or Windows as their day to
+day operating system. This means that the ``environment.yml`` created by a strict
+export will not work with error messages saying that certain packages can not
+be resolved (``ResolvePackageNotFound``).
+
+
+The solution
+============
+
+Follow this procedure to create a strict export of your environment that will
+work with ``repo2docker`` and sites like `mybinder.org <https://mybinder.org/>`_.
+
+We will launch a terminal inside a basic docker image, install the packages
+you need and then perform a strict export of the environment.
+
+#. install repo2docker on your computer by following :ref:`install`
+#. in a terminal launch a basic repository
+   ``repo2docker https://github.com/binder-examples/conda-freeze``
+   inside repo2docker
+#. open the URL printed at the end in a browser, the URL should look like
+   ``http://127.0.0.1:61037/?token=30e61ec80bda6dd0d14805ea76bb59e7b0cd78b5d6b436f0``
+#. open a terminal by clicking "New -> Terminal" next to the "Upload" button on the
+   right hand side of the webpage
+#. install the packages your project requires with ``conda install <yourpackages>``
+#. use ``conda env export -n root`` to print the environment
+#. copy and paste the environment you just printed into a ``environment.yml`` in
+   your projects repository
+#. close your browser tabs and exit the repo2docker session by pressing Ctrl-C.
+
+This will give you a strict export of your environment that precisely pins the
+versions of packages in your environment based on a linux environment.

docs/source/index.rst

@@ -32,6 +32,7 @@ Please report `Bugs <https://github.com/jupyter/repo2docker/issues>`_,
 
    howto/user_interface
    howto/languages
+   howto/export_environment
    howto/lab_workspaces
    howto/jupyterhub_images
    howto/deploy