Move to explicit simtools-prod image tags in gitlab CI

The parameter file validation step in the gitlab IC requires a simtools-prod image. Unfortunately the gitlab pull policy is only pulling a latest image once a day (I think so), and we didn't manage yet to convince gitlab admins to allow us to change these settings.

As latest is anyway not a 'good concept', my suggestion is to explicitly add the tag of the simtools-prod package to the gitlab CI. This requires additional one-line changes, but I thing this is ok.