Use a Debian Stretch base image

What does this MR do?

Make sure we don't switch to a Debian Buster based image. Ruby can pin to Stretch, so let's do that for the next couple of months, to avoid the bug described in https://gitlab.com/gitlab-org/gitlab-ee/issues/13830.

Note that the current docker image is NOT affected, because it was built before the switch to Buster. If we were to rebuild it today, it would fail with docker 19.03.00.

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