typo correction on env variable: replacing $USER with $NB_USER since $USER is not set
test if group exists before trying to add a new group

Workaround of a Docker bad handling of sparse files
Fixes #923

bump Julia version (1.1.0 => 1.1.1)

Update all package versions

Option to enable restart without exiting container

- New versions found with conda --upgrade

Add optional environment variable RESTARTABLE to enable Jupyter to
be restarted without causing the container to exit.  When the variable
is set, run-one-constantly (from the Ubuntu run-one package) is
inserted into the Jupyter startup command.

Switch to matplotlib-base to avoid qt dependency

Use prod instead of dev build for JupyterLab

Bump notebook to 6.0.0, jupyterlab to 1.0.4, conda to 4.7.10

Inline PNG breaks sphinx gettext

Add java-notebook to the list of community stacks.

I modified 'docs/using/selecting.md' to include a link to the java-notebook project.

Added install of e1071 R package

conda 4.7.5

jupyterlab_bokeh@1.0.0 (rebase on PR #894) for JupyterLab 1.0.0

it makes the conda env invalid, which causes errors in later installs

Community haskell stack binder link

has fixed dependencies

much faster installs

had to move WORKDIR $HOME up due to tempfile permission issues with conda 4.7

Pin conda version to $CONDA_VERSION

Pin the conda version so that a call to `conda update --all` will not
cause conda to update itself to a higher version with possible breaking
changes.

Motivating example: Upgrade from __4.6.*__ to __4.7.5__ causes the build to fail
due to a conda error:

    [Errno 13] Permission denied: '/.cph_tmpilpdzq7x'

Resolves #891

Add binder link to Community Stack crosscompass/ihaskell-notebook.