Force tornado 5 because version 6 breaks jupyter

Dockerfile

@@ -79,7 +79,9 @@ RUN curl -O https://bootstrap.pypa.io/get-pip.py && \
     python3 get-pip.py && \
     rm get-pip.py
 
+# Force tornado 5 as version 6 breaks jupyter
 RUN pip3 --no-cache-dir install \
+            'tornado==5.1.1' \
             'notebook==5.7.4' \
             'jupyterhub==0.9.4' \
             'jupyterlab==0.35.4' \