Support workspace images to be pulled from private registries

Summary

Starting 17.6, there is support to use images from private container registries. However the supporting images for workspace are not pulled from the private repo. For example alping/git image is always pulled from docker.io.

Steps to reproduce

1. Create a workspace using GitLab agent.
2. You will notice that alpine/git:2.45.2 image gets pulled from docker.io.

What is the current bug behavior?

If image_pull_secrets is configured, images are not pulled from the repositories defined in the dockerconfig secret.

What is the expected correct behavior?

If image_pull_secrets is set for remote development, try to pull images from the repository from the docker registry secret.

Relevant logs and/or screenshots

Normal   Pulling    2s (x3 over 17s)  kubelet            Pulling image "alpine/git:2.45.2"

Output of checks

Possible fixes