Update Container Scanning documentation

What does this MR do?

This MR updates doc/ci/examples/container_scanning.md so that it gives correct code that avoids infinite loops when the Clair scanner server doesn't start.

Are there points in the code the reviewer needs to double check?

None, this is the same code as the vendored one in doc/ci/examples/container_scanning.md

Why was this MR needed?

To update outdated doc.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • [ ] Has been reviewed by UX
  • [ ] Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• [ ] Conform by the merge request performance guides
• [ ] Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered
• [ ] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45543 (closed)