docker: add Python3.6 to image

docker: add Python3.6 to image

This is needed to support py3.6-django111 in tox - otherwise tox
fails.

Python3.6 is not in Xenial, so this - irritatingly - requires pulling
in either a PPA, or the package from Artful. PPAs are icky, so pull it
in from Artful.

We can either add an Artful repo (like we do for Trusty to pull in
python3.4), or move the image to Artful and add in the Xenial repo
to get python3.5. It's more efficient to move the entire image to
Artful and pull in 3.5 from Xenial - otherwise most packages get
downloaded from Artful anyway. It does mean we're going to need to
move again to Bionic in a few months, but we'll just have to deal.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>