Use debian/ubuntu for docker images
Currently we use alpine for our docker images. This is an issue because
- It is different from what we use everywhere else (README, gitlab-ci, developer's machines)
- It is slow (see !1824 (closed))
I already tried to port everything in !1454 (closed). The thing that stopped me was cron configuration (see also !1290 (merged)). So I need help with that.
So we need to update !1290 (merged) and fix the cron configuration. After that we should be able to also merge !1824 (closed).