upgrade to tensoflow2

Add maintainer help wanted

update jupyterlab to 1.2.1

fix typo

With JupyterLab 1.1, --dev-build=False is no longer necessary.

Make fix-permissions executable by all

Don't upgrade base image

Update Miniconda to 4.7.10 and conda to 4.7.12

Address updating base notebook to jupyterlab 1.1.3

Add link to sage-notebook Community Stack

Bump Julia to 1.2.0

Added sage-notebook as per https://github.com/jupyter/docker-stacks/blob/master/docs/contributing/stacks.md

Sort package installation lines alphabetically for easier comparison

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