This reverts commit 02e51284.

* Try to prevent the container from restarting all the time by adding 
`-f` to `crond`
* `touch /crontab.conf` not needed
* `cat /crontab.conf` not needed because `start-rotation.py` already 
prints the config

/etc/crontab does not exist in Alpine. No idea why the `touch 
/crontab.conf` was added.

... and CVE-2019-17455 from clair-whitelist.yml.

These three vulnerabilities seem to be fixed with the new Alpine image

CI seems to have started from the wrong path in previous build: 
https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/jobs/883015

CVE-2020-8492 reported by Clair in previous build: 
https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/jobs/883005

This comes from 'libpython2.7', which is one of many unnecessary 
dependencies that get installed with 'cron' dy default

Revert this commit to enable backups again.

Cf. 21c1e819