CI: optimize package installation
It seems that new Docker image introduced in !375 (merged) is not fully used because e.g. https://gitlab.labs.nic.cz/knot/knot-resolver/blob/master/.gitlab-ci.yml#L47 is still doing some package installation etc. which slows whole CI down.
Most of the package preparations should be done in Dockerfile so the CI just runs the image and we do not spend time on waiting while running CI jobs.