Where do you host the base images? Someone's footing that bill or are you paying for it?
Often, there isn't time for an image to be pulled from a mirroring serving.
There again there's the question of finding image sources you can trust to be updated and secure. Docker Hub has a "Docker Official Image" tag for critical base images that are managed by each community.
Re. finding the sources, a central namespace may still be useful, hence the use of caching proxies.