* 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.

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

Correctif Dockerfile (la copie d'un fichier utilisée pour des tests n'est plus utile depuis que l'image a été ajouté au docker-compose)

README.md added, written in markdown. backup_data.json removed. start_rotation.py modified to check for exceptions and close the config file correctly. Comments written in english dor both scripts.