Clean of the Dockerfile

I cleaned up the Dockerfile of some commented instructions that aren't
relevant anymore and as well some unecessary packages.
Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>

django/Dockerfile

 FROM debian:stretch
 
-# install system packages
-# Installation of gunicorn through pip because the debian package
-# python3-gunicorn doesn't install any binary
-
 # Install Stretch backport
 RUN echo "deb http://ftp.debian.org/debian stretch-backports main" \
       >> /etc/apt/sources.list.d/stretch.list
@@ -12,7 +8,7 @@ RUN apt-get update && apt-get -t stretch-backports install -y \
       python3-django python3-django-allauth
 
 RUN apt-get install -y  mysql-server default-libmysqlclient-dev \
-  nginx-light zip	python3-dev python3-pip python3-mysqldb python-mysqldb \
+  nginx-light zip	python3-dev python3-pip python3-mysqldb \
   supervisor && pip3 install gunicorn