fix: use supervisor

While checking if the cron tasks were performed properly on my own cluster... I figured out that they were actually not started.

Indeed when executing exec docker-entrypoint.sh, I thought that the PostgreSQL process was (somehow) preventing cron from running (which now I know is not the correct reason). So I spent some time integrating supervisor to the Docker image (and it works well for multiple bash files, we can make them run in parallel easily!). But cron was still not working. In the end I figured out that the cron file used was system-wide, and that one entry was missing a username... So I fixed it and it worked. This fix is integrated to the main branch and supervisor becomes useless