If a choir teacher distributes the lyrics to a Britney Spears song to their students for practice, there is nothing illegal about this
If a choir teacher starts a website britneylyrics.com and puts ads on the website, that would qualify
The EULA might prohibit redistribution, but you don't need to accept an EULA to copy-paste files, as far as I know.
> The EULA might prohibit redistribution
I don’t think it matters. Copyright law automatically forbids copying. Well, assuming Apple complied with any requirements to have a valid copyright, which seems a safe bet.
You can extract the images yourself from official install media (for instance, the installers you can create from within macOS) and use it for whatever personal project you want; you'd be breaking the EULA, but that doesn't mean much. You're not allowed to throw your copy on the internet, though.
Other projects I've seen download the installer images directly from Apple, something they could probably detect and block if they wanted to. That would probably be completely legal, as nobody is unlawfully distributing the files. This is different; the Docker images contain a copy of macOS.
Apple could probably take this project down any time they want to, but if they wanted to they probably would've already.