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

Issue

In an incident that occurred on November 30th (3AM UTC+3) that affected the GitLab Container Registry, images pulled from there could not be accessed.

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. 

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.

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. This would provide an alternative for Self-Managed clients who depend on GitLab's Container Registry to access the images.

Possible Alternatives

These alternatives were mentioned by the customer

1. Setting up Pull Policies
2. Have a secondary location where these images are published to
3. Mirroring images from gitlab.com

Context:

• This issue was raised by an affected by a Large Premium Customer (Internal Only).