Add documentation about registry mirror in dind

What does this MR do?

Docker Inc is rolling out new terms of service which will introduce rate limits on docker pull from DockerHub for anonymous users.

Users can already set a DockerHub mirror to not reach these limits when running CI jobs using the Docker executor. However, when they use the dind service to build their images this won't use the registry mirror because it needs to be explicitly configured for the daemon.

Related issues

gitlab-runner#27171 (closed)

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