docker.io pull limit breaks runners

(sorry for the short report, will try to expand later)

Summary

docker.io put in place a pull limit for free users, and it's breaking the docker runners.

Actual behavior

When running a job (in my case, kubernetes executor) it fails while pulling the gitlab-helper-image.

Wanted to create an issue to track this as I expect many projects will start to have the same problem

Expected behavior

By default, the helper_image should be the one hosted in registry.gitlab.com, or at least document the helper_image parameter correctly (it's flagged as "Advanced")

https://docs.gitlab.com/runner/configuration/advanced-configuration.html#helper-image

Relevant logs and/or screenshots

ERROR: Job failed (system failure): prepare environment: image pull failed: rpc error: code = Unknown desc = Error reading manifest x86_64-6fbc7474 in docker.io/gitlab/gitlab-runner-helper: toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit. Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information