[META] Improve triggered package builds

Second iteration of https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2234 .

This issue is used to track the required changes to make further improvements on triggered package builds from CE/EE repositories.

• Better visibility of the triggered pipeline: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2318 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11188
• Use caching for triggered builds https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2319
• Reduce size of the Docker image https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2320 - https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1537
• Automatic cleanup of stale images https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2321
• Specify omnibus branch on triggered build https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2323
• Trigger a build from the omnibus project https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2330 - https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1539
• Dedicated infrastructure for building triggered packages
• Triggered packages ran by default
• Use multi-project pipelines

/cc @gitlab-build-team @joshlambert

mentioned in issue #2330 (closed)

marked the checklist item Better visibility of the triggered pipeline: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2318 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11188 as completed

marked the checklist item Trigger a build from the omnibus project https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2330 - https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1539 as completed

marked the checklist item Reduce size of the Docker image https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2320 - https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1537 as completed

It would be great if the Docker Image tag could be displayed at the end of the job. For now, we have to search for Successfully built sha.

Do we want to know who triggered a build? We can pass it as a variable to the trigger and display it on log.

I don't see why not.

changed the description

mentioned in issue #2581