gitlab-ci: use server image from https://gitlab.com/fdroid/ci-images
gitlab-ci: use server image from https://gitlab.com/fdroid/ci-images

This image provides everything needed to run all aspects of fdroidserver,
and is only like 100 megs bigger than the base image that this was using
before.

f-droid/fdroiddata,f-droid/fdroid-data,f-droid/fdroiddata