* 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

This reverts commit b619ab99.

Vulnerabilities were deleted from wrong folder X)

Also, update some comments.
Line 50: `- "pica-ci-base/**"` changed to `- "pica-ci-base/Dockerfile"` 
in order to match comment

Bad idea to use docker_test.sh, it would require too much modifications 
just for this special case

In order to avoid unnecessary pipelines

https://docs.gitlab.com/ce/ci/yaml/#rules

... and add example 'htpasswd' file

(and with spaces it's easier to copy-paste in a console for debugging)

...'build', 'clair' and 'docker-bench-security'

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

These vulnerabilities were also fixed by 5b6bd312

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

This change was already in prod since some days, but not commited