Have CI build a container which contains exactly what we need in
production, and test that.

On top of the base container for prod, we also have Selenium testing,
and tox. Neither of them are appropriate to have in the production
container: we don't want to install a browser on our web server, and we
also don't want the separate venv that tox installs. But they are still
good tests to run, so we run those in a separate job.