Move docker images used by release/test infrastructure into a separate project

[jlewi]

We use gcr.io/kubeflow-ci/test-worker as the worker image in our CI/CD workflows.
We would like to use the same Docker image in our release workflows.
But we'd like to make sure untruted code running in PRs can't modify those images.

So it would be good to move these docker images into a separate registry such that code running in kubeflow-ci can read but not write that registry.

[stale]

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[stale]

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