... 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

Revert this commit to enable backups again.

Cf. 21c1e819

Revert this commit to enable backups again.

Backups were run every six hours, but due to the huge increase in the 
size of our DB (Docker volume 'etherpad-db' takes 47 GB at this time), 
backups take almost 3h30 to complete, slowing down the entire instance 
during that period.