Docker images should be regulary scanned for vulnerabilities in Python modules

[btlogy]

Since #27, we can keep track of some vulnerabilities in the base images by getting automatic pull request via Dependabot.

But we are still blind to the known vulnerabilities present in the Python modules installed on top of those images.

In addition, the current update process may lack of re-reproducibility: are we sure we do install all the same modules version each time we build the Debian images?

REM: @wuan is already working on #30 based on a discussion we had about that on Slack.