We do this for most of our docker image builds in CI. Definitely worth it if your CI builds take significantly longer than it takes to pull down the image layers from previous builds (sometimes network speeds can end up slowing it down with a method like this)
This article is weirdly vague in the sense they didn't mention the registry is is an official docker container: https://hub.docker.com/_/registry
It also doesn't go into any of the detail about starting up / configuring the registry, or explaining that running your own version is like running you own private hub.docker.com.
The registry is great! It just doesn't seem to be maintained much anymore, maybe because it's feature complete, or maybe because Docker is trying to sell plans to their "hub".
For anyone else interested in advanced Docker features: check out Docker Swarm (the only way to reasonably run Docker Swarm is to manage your own Docker Registry).
I already have a docker registry too, so I guess I missed the point of your entire article. You can use the registry as a cache for build artifacts... that's what it's for. Your article was written like you just figured that out, and I was providing context to the other confused HN users.