Propogate pipeline vars

What does this MR do?

Propagate PUBLISH_IMAGES and other vars set in the scheduled pipelines of the gemnasium project, or set manually.

Right now the Docker images can't be published this way, b/c PUBLISH_IMAGES is never passed to the child pipelines, where we have the release jobs.

See https://docs.gitlab.com/ee/ci/yaml/#triggerforward

What are the relevant issue numbers?

None

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer