improve get_image_refs.py to manage images_with_unknown_unit list

This issue proposes enhancements to the get_image_refs.py script to improve the handling of the "images_with_unknown_unit" list. The goal is to ensure that this list is minimized or eliminated, thereby enhancing the accuracy of image-unit mappings.

As mentionned by @tmmorin in this comment we should consider implementing something like:

• Allow get_image_refs.py to accept a config file with regex rules for matching container image names and their associated units
• Add a flag to get_image_refs.py to have it fail if images_with_unknown_unit is not empty (not enabled by default)

We would incrementally improve the list of rules to reduce images_with_unknown_unit.
Once we have an empty images_with_unknown_unit in our CI runs, we would turn on the "fail if images_with_unknown_unit is not empty" flag in CI