Yep, a MANIFEST file would be also a very good idea.
But there should be a build-MANIFEST and a binary-MANIFEST, as the dependencies are different if you build from source and if you download the binary.
The build-MANIFEST could be just a link to root-ci-images/ubuntu2404/packages.txt at main · root-project/root-ci-images · GitHub
But this file should be branch-specific and contain OS-specific links.
The same for the website, a link would avoid having to update things in two places.