Fix allowed_images behavior

Merged Tomasz Maczukin requested to merge fix/regression-in-docker-executor into master

What does this MR do?

Fixes regression introduced while working on !596 (merged)

Why was this MR needed?

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

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Fixes #2624 (closed)