ci: mark docker image jobs and pre-commit checks job as placeholder jobs
requested to merge tpm/cerbero:ci-mark-docker-image-jobs-and-pre-commit-checks-job-as-placeholder-job into main
They finish fairly quickly in most cases and even if they need to rebuild the image they won't use a lot of cpu resources.
This way these jobs don't have to compete with the normal job queue and things get started a bit quicker.