docker: Install dependencies for Python 2 also

docker: Install dependencies for Python 2 also

Aspects of both Patchwork and Django, such as the 'makemigrations'
management command, function differently under Python 3. It should be
possible to run these commands under Python 2. Install dependencies in
the Python 2 environment to make this a possibility.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Acked-by: Daniel Axtens <dja@axtens.net>