It seems like it would be super easy for them to close this loophole. If you use the model that free tier listeners (real ones) will listen to about the same distribution of songs as the paying listeners, then just stop counting all free tier listeners and multiply the amount paid out for the pay-tier listeners by an appropriate factor to make payouts the same as before.
Folks, the docker runtime is open source, and not even the only one of its kind. They won’t charge for that. If they tried to make it closed source, everyone would just laugh and switch to one of several completely free alternatives. They charge for hosting images, build time on their build servers, and various “premium” developer tools you don’t need. In fact, you need none of this, you can do all of it yourself on whatever hardware you deem to be good enough. There are also many other hosted alternatives out there.
Docker thinks they have a monopoly, for some reason. If you use the technology, you are probably already aware that they don’t.