Add a local (~s3) Docker registry in hosted images (for build caches)

Currently we have a local `bazel-remote` server in the hosted images that proxies to an s3 bucket that allows storing action caches and gives (non-RBE) CI a massive optimization

We could do something similar with docker/buildx which allows you to `cache_to`/`cache_from` build caches to/from a registry which we could run with a bucket storage backend

This would speed up/reduce cost a lot for docker-centric CI jobs and would avoid much more complicated and less reliable azp caching approaches.