Document alternative to finding helper images other than the GitLab Container Registry

Issue

In an incident that occured on November 30th (3AM UTC+3) affected the GitLab Container Registry.

Based on the Helper Images documentation, Helper images are only published in GitLab Registry:

In GitLab 15.0 and later, the helper image is pulled from the GitLab Container Registry. In addition, helper images are only published in the GitLab Container Registry.

One of the effects was that helper images in the Container Registry couldn't be pulled and had errors such as:

 failed to authorize: failed to fetch anonymous token: unexpected status: 502 Bad Gateway

and

failed to resolve reference "registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helperxxxx": failed to authorize: failed to fetch anonymous token: unexpected status: 502 Bad Gateway. 

Proposal

Provide an alternative to pull Helper Images and any other images that are only pushed to GitLab's Container Registry and document this alternative.

Context: