Use a Debian Stretch base image

What does this MR do?

Lock to Debian Stretch image, to avoid the bug described in https://gitlab.com/gitlab-org/gitlab-ee/issues/13830

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/13830

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