Reproduce private pulling issue + poc a fix

[jhchabran]

Follow-up to #126 (comment)

I spent some time exploring to understand if something was wrong on my side or if there was an issue and managed to pull private images with the crude fix that'll find here.

The code that handles the authentication seems to make assumptions about ~/.docker/config.json that do not match the structure of that file.

# ~/.docker/config.json 
{
  "auths": {
    "https://index.docker.io/v1/": {}
  },
  "credHelpers": {
    "asia.gcr.io": "gcloud",
    "eu.gcr.io": "gcloud",
    "gcr.io": "gcloud",
    "marketplace.gcr.io": "gcloud",
    "staging-k8s.gcr.io": "gcloud",
    "us-central1-docker.pkg.dev": "gcloud",
    "us.gcr.io": "gcloud"
  },
  "credsStore": "desktop"
}

But the original code tries to look for us.gcr.io if that's what I set to the registry attr on the oci_pull. It then takes a look at the credsStore which will is set to desktop and doesn't know how to find my private image.

The fix is to instead iterate on the credHelpers if we don't find a matching entry under "auths", which I very crudely implemented.

I have included in my commit the modifications I did to test the code locally, which allowed me to successfully pull the image.

At this stage, I think it's best for me to reach out, as I don't know what would be the best solution to write a proper e2e test for such a patch. With some guidance, I could implement the fix properly, though I'm totally okay if you want to take over.

[thesayyn]

closing in favor of #237