Option to enable restart without exiting container

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.

base-notebook/Dockerfile

@@ -24,6 +24,7 @@ RUN apt-get update && apt-get -yq dist-upgrade \
     sudo \
     locales \
     fonts-liberation \
+    run-one \
  && rm -rf /var/lib/apt/lists/*
 
 RUN echo "en_US.UTF-8 UTF-8" > /etc/locale.gen && \

base-notebook/start-notebook.sh

@@ -4,11 +4,16 @@
 
 set -e
 
+wrapper=""
+if [[ "${RESTARTABLE}" == "yes" ]]; then
+  wrapper="run-one-constantly"
+fi
+
 if [[ ! -z "${JUPYTERHUB_API_TOKEN}" ]]; then
   # launched by JupyterHub, use single-user entrypoint
   exec /usr/local/bin/start-singleuser.sh "$@"
 elif [[ ! -z "${JUPYTER_ENABLE_LAB}" ]]; then
-  . /usr/local/bin/start.sh jupyter lab "$@"
+  . /usr/local/bin/start.sh $wrapper jupyter lab "$@"
 else
-  . /usr/local/bin/start.sh jupyter notebook "$@"
+  . /usr/local/bin/start.sh $wrapper jupyter notebook "$@"
 fi

docs/using/common.md

@@ -33,6 +33,7 @@ the notebook server. You do so by passing arguments to the `docker run` command.
 * `-e GRANT_SUDO=yes` - Instructs the startup script to grant the `NB_USER` user passwordless `sudo` capability. You do **not** need this option to allow the user to `conda` or `pip` install additional packages. This option is useful, however, when you wish to give `$NB_USER` the ability to install OS packages with `apt` or modify other root-owned files in the container. For this option to take effect, you must run the container with `--user root`. (The `start-notebook.sh` script will `su $NB_USER` after adding `$NB_USER` to sudoers.) **You should only enable `sudo` if you trust the user or if the container is running on an isolated host.**
 * `-e GEN_CERT=yes` - Instructs the startup script to generates a self-signed SSL certificate and configure Jupyter Notebook to use it to accept encrypted HTTPS connections.
 * `-e JUPYTER_ENABLE_LAB=yes` - Instructs the startup script to run `jupyter lab` instead of the default `jupyter notebook` command. Useful in container orchestration environments where setting environment variables is easier than change command line parameters.
+* `-e RESTARTABLE=yes` - Runs Jupyter in a loop so that quitting Jupyter does not cause the container to exit.  This may be useful when you need to install extensions that require restarting Jupyter.
 * `-v /some/host/folder/for/work:/home/jovyan/work` - Mounts a host machine directory as folder in the container. Useful when you want to preserve notebooks and other work even after the container is destroyed. **You must grant the within-container notebook user or group (`NB_UID` or `NB_GID`) write access to the host directory (e.g., `sudo chown 1000 /some/host/folder/for/work`).**
 * `--user 5000 --group-add users` - Launches the container with a specific user ID and adds that user to the `users` group so that it can modify files in the default home directory and `/opt/conda`. You can use these arguments as alternatives to setting `$NB_UID` and `$NB_GID`.