Add test for postBuild script
ADD
tests/venv/postBuild/postBuild
+#!/bin/bash
+jupyter nbextension enable --py --sys-prefix ipyleaflet

ADD
tests/venv/postBuild/requirements.txt
+ipyleaflet

ADD
tests/venv/postBuild/verify
+#!/bin/bash
+set -e
+jupyter nbextension list | grep 'jupyter-leaflet' | grep enabled

Add test for postBuild script