CI: use selenium service for inside tests

For the docker-inside tests run, we install and use a chromedriver installed in the Heptapod Docker image.

For testing of Heptapod 0.20, there's been a complication: the Heptapod image is now based on Ubuntu 20.04, for which all Chromium related software is provided as Ubuntu snaps (can't work in our CI context). We had to install it from a PPA. fix a few things.

But the point of these runs is for the functional tests to access the file system directly and manage the Heptapod services with gitlab-ctl. Nothing should prevent us to use an external service for Selenium, as we do for the remote tests run. We'd get

• more uniformity
• faster starting time, with the natural Docker image cache (installing chromedriver is long)
• independency from the base software of the Heptapod Docker image