In order to have an up to date version of Python and therefore an up to
date pip version, the Python Docker image is used instead of the Debian
one.
The choosen Python Docker images are based on desired version of Debian.
This avoid to upgrade pip inside the image, that was resulting in a
useless copy of pip.
While phonenumbers is not required to run Odoo, it's a requested nice to
have.
Upgrading pip solves a red warning issued when pip installs docopt.
closes#245 and closes#168
