Add the db check for the containers

Since we have set the database to be in auto-commit mode, we can make a
check of the connectivity between the web container and the db.

If the database is set to a transactional level (not in auto-commit
mode), this ping() check is problematic in the sense that the
transaction increase at each check (because of the `select` SQL
command), and the entire transaction will crash at some point.