fix: Use credentials from DOCKER_AUTH_CONFIG

Open username-removed-1529220 requested to merge mikebryant/gitlab-runner:wip-2673 into master

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What does this MR do?

Uses DOCKER_AUTH_CONFIG if available, preferring it over the other credentials

Why was this MR needed?

Currently DOCKER_AUTH_CONFIG is ignored, meaning that you can't pull from a private registry on Kubernetes

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 #2673

Edited by username-removed-1529220