Pull gitlab runner helper image from GitLab registry by default

Overview

With !2554 (merged) we introduced a feature flag FF_GITLAB_REGISTRY_HELPER_IMAGE which is turned OFF by default. When this feature flag is turned ON it pulls the helper image from registry.gitlab.com instead of DockerHub.

Proposal

When we see that everything is working correctly on GitLab.com $ISSUE_ID and wait for any customer feedback, we should have FF_GITLAB_REGISTRY_HELPER_IMAGE ON by default to pull images from registry.gitlab.com instead.