Push prebuilt images to dockerhub

Merged Tomasz Maczukin requested to merge improvement/dockerhub-runner-helper into master

What does this MR do?

Adds a release step, that pushes gitlab-runner-helper do docker hub. This is a first step to make prebuilt images available for Kubernetes executor.

Why was this MR needed?

Please read !275 (closed) for a reference.

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?

Successor of !275 (closed)