This isn't too surprising. Microsoft's DevOps in Azure does the same thing (or did I haven't looked at it in a few months). There was literally no point in using it until, as you've pointed out, a user can leverage cache. If I have a multistage build with an SDK that weighs in around 1GB why would I ever want to use a tool that pulls that down every run?

I think, as many have said, that this is going after GitLab more than anyone else, although I can see a lot of users migrating away from Docker Hub given 1) the latest snafu/breach and 2) why keep my container repo over here and my container build pipeline over there? Doesn't make any sense and Docker Hub doesn't come with the pedigree of CDN baked in. I'm sure the same arguments work for other technologies in this consideration, but... Docker seems to continually be behind the 8-ball on the shifting field. My guess is Microsoft buys them in the next 3 years at a discount anyway. It fits their pattern of getting in front of the modern ecosystem and since Docker has leverage with containerd right now it would be an unsurprising move.