Add SEC_REGISTRY_IMAGE for v2-FREEZE

What does this MR do?

Add SEC_REGISTRY_IMAGE variable for v2 FREEZE branch which along with the deploy token added to the settings will allow this project to start pushing images to a more locked down Docker registry.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab/-/issues/336372

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