Yeah, very similar approach. We did this before, see e.g. https://www.stablebuild.com/blog/create-a-historic-ubuntu-pa... - but then figured everyone needs exactly the same packages cached, so why not set up a generic service for that.
Here's a related discussion about reproducible builds by the Docker people, where they provide some more details: https://github.com/docker-library/official-images/issues/160...