Using python 3.5 image to build the doc

The image also have make installed, so we just need to install sphinx. Sphinx image also exists, but less official so I prefered using python:3.5 as in https://gitlab.pasteur.fr/hub/12765-viralhostrangedb/blob/master/.gitlab-ci.yml#L133-145

mentioned in merge request !3 (merged)

merged

mentioned in commit c5b662c9