Hosting copyrighted media without a distribution license is usually illegal. Very few countries allow you to just distribute proprietary disk images like this.
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.
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.