config.toml docs do not actually describe how to setup a docker private registry

The relevant section of documentation is here: https://docs.gitlab.com/runner/configuration/advanced-configuration.html#using-a-private-container-registry

It begins by saying:

If you want to use private registries as a source of images for your builds, you can set the authorization configuration in the DOCKER_AUTH_CONFIG variable. It can be set in both GitLab Variables section of a project and in the config.toml file.

But it never goes on to say what to put in config.toml.