support private registries with Kubernetes
What does this MR do?
Appears private registries work with Kubernetes using DOCKER_AUTH_CONFIG
secret variable even though docs say they are not.
Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1957
Might have been fixed by https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/520?