I updated the packages installed in order to:

- remove some unecessary packages related mainly to Rails
- add packages relation to python 3 and Django

I updated the container initialisation that seems to fit the current
practice with other container initialisation process.

One thing to mention. Supervisor can't work with Python 3. Anthony
suggested to install Python 2 just to make Supervisor happy and run
Gunicorn/Python 3 as such.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>