Single docker 3

New MR because !1447 (merged) was merged by accident.

Yet another base image after !5 (merged), !14 (merged), and !40 (merged).

• Use ubuntu instead of alpine (same as in gitlab-ci and README)
• Do not include recommended packages with apt
• No longer uninstall build dependencies as it has basically no effect

The new image is 727MB (compared to 873MB before)

Open questions:

• What to do about cron? For now I added busybox to use the same crond as before but I am not really happy with the logging. Ubuntu already include a "regular" cron daemon so it seems silly not to use it. See also !1290 (merged).
• How to create the uwsgi user? I tried --system but then django was unable to write to /log/. I feel like there should be a simpler version than what I did.

@krause could you have a look?