...or clair-whitelist.yml

* 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