Aye, It's a bit like saying you can't sell your code, because you wrote it in someone elses software.

Writing a decent Dockerfile isn't hard, and keeping it maintained and working with new versions is still work and it's past the wheelhouse of very many people. It's entirely reasonable to want paid for that effort.

That said, it's not work I personally value enough to put my hand in my pocket, and that's a fair take too.