Fix Docker build rules to use $CI_DEFAULT_BRANCH

The build rules were checking for 'main' branch, but this repo uses
'develop' as the default branch. This caused builds to never run on
develop because:
- Rule 1 checked for 'main' (never matched)
- Rule 2 used compare_to: refs/heads/develop, which shows no changes
  when already on develop

Changes:
- Replace $CI_COMMIT_BRANCH == "main" with $CI_DEFAULT_BRANCH
- Replace regex !~ /^main$/ with != $CI_DEFAULT_BRANCH
- Add allow_failure: true to buildkit_cache_cleanup (cleanup shouldn't block pipelines)