CI: Fix hadolint issues

Now that we have hadolint as part of our CI pipelines, we should address the issues it reports.

Here's an Aug 24 pipeline.

As of today, the amount of issues reported is minimal. If we do not want to do what hadolint suggest, we should selectively disable its specific warnings.

Once the pipelines are clean, we should enable the "pipelines must succeed" option for this project.