ci(images): Reduce builds by only check for changes from default branch

This patch tries to use the GitLab CI feature that allows to
detect changes from a specific branch, in this case the default
branch, and this way hopefully reduces the amount of
unnecessarily build images that originate from gitlab starting
a full build by default on a new branch.

References:
https://docs.gitlab.com/15.4/ee/ci/yaml/#ruleschangescompare_to