GitLab CI Kubernetes Runner does not honor DOCKER_AUTH_CONFIG

Summary

GitLab CI Kubernetes Runner does not honor Docker authorization configuration for private registries in DOCKER_AUTH_CONFIG. It seems that setupCredentials function in Kubernetes executor uses only authentication credentials from the job payload.

Steps to reproduce

1. Set up GitLab Runner to use a Kubernetes cluster.
2. Pick any image in a private Docker registry which is NOT accessible implicitly from the Kubernetes cluster (for example, don't use gcr.io when running Kubernetes on Google Cloud, or azurecr.io when running on Azure).
3. Try to run a job using that image while passing an appropriate DOCKER_AUTH_CONFIG environment variable.

Actual behavior

Image pull fails when starting the job pod.

Expected behavior

Image pull should succeed.

Relevant logs and/or screenshots

Running with gitlab-ci-multi-runner 9.4.1 (d24b11c)
  on Kubernetes Runner (6066fd46)
Using Kubernetes namespace: gitlab-ci
Using Kubernetes executor with image $BUILDENV_IMAGE:$BUILDENV_KEY ...
Waiting for pod gitlab-ci/runner-6066fd46-project-29-concurrent-0s2qc6 to be running, status is Pending
Waiting for pod gitlab-ci/runner-6066fd46-project-29-concurrent-0s2qc6 to be running, status is Pending
Waiting for pod gitlab-ci/runner-6066fd46-project-29-concurrent-0s2qc6 to be running, status is Pending
ERROR: Job failed: image pull failed: Back-off pulling image "<snipped>"

Environment description

This is a on-premise installation of GitLab. GitLab Runner is deployed into a Kubernetes cluster on Azure. The image is not from an Azure Container Registry, which is available without any manual authentication in this cluster.

Proposal

• At the moment all the logic around authentication is inside of the Docker executor. Extract the authentication into a common library just like what we tried in !1164 (diffs) keep in mind !1164 (comment 161663797). This should be a package under https://gitlab.com/gitlab-org/gitlab-runner/-/tree/master/helpers/docker maybe it's own package auth which will be responsible of all the authentication.
• When there is a common library that we can have both the Docker and Kubernetes executor use it.