This should help people understand a bit better why this feel a bit underhanded. The images are free, and I and many other OSS devs have used docker hub in partnership to provide access to software, often paying for the ability to publish there. In this case, any burden of extra cost was on the producer side.
Turning this into a way to "know" every user and extract some value from them is their prerogative, but it does not feel like it is good faith. It also feels a bit creepy in the sense of "the user is the product".