You can create your own proprietary OS and never even sell it or use in your machines... It's yours and unless you authorize, others can't distribute it.
For Docker to decide to remove the image from their website? Obviously not.
Docker runs the service and can do whatever they want.
If Apple sends a cease and desist, and Docker decides they'd rather take it down than risk going to court with Apple /to determine if there were damages/ - then they're free to do so.
The other reasonable course of action would be to go to court, where a jury of average citizens could determine whether or not Apple or Docker is in the wrong and decide damages.
Docker wasn't pushed into taking the image down, they *decided* that they'd rather take the image down than have a jury decide who is in the wrong.
that's very important to point out yes! it's not like apple software products are covered by copyright law after all. it's the big bad lawyers that's ruling the day here.