RPM is a packaging format, not a packaging standard. Fedora has a massive set of guidelines. OpenSUSE has a massive and different set of guidelines, because it's a different distro with different goals. Mandriva, another set. And so on. The fact they all happen to use RPM is pretty much irrelevant.

This is true, but they're also all Linux distributions providing exactly the same software, and all duplicating hundreds of hours of effort per package by doing exactly the same thing with tiny incompatible differences. It doesn't have to be that way. For example, they could provide shims for the incompatibilities in each others' standards so the packages edit: spec files could be reused without breaking on other systems. But instead they work in silos. Sigh

This is a problem (if you consider it a problem) of all Linux distros. It's just that people look at the foo.rpm file and think it should be installable anywhere, but no one would look at a foo.deb file and think they could install it on Arch.

If Debian packages only required a single text file to build everything, they could! It would be simple for the distros to combine their efforts on a single .spec file. Back in the day I used to make RPM .specs that would build the package successfully on Solaris, Linux, Windows, etc and then had tools to convert the RPM into a native package format for easier maintenance. Writing an rpm2dstream script took just a couple hours.

A (really old version of) RPM is in the LSB. OTOH the LSB is dead - log into any Linux box and run lsb_release

THe .src.rpm is not required if you use rpmbuild (which is what docker-rpm-builder does inside the container) but AFAIK you need to build a .src.rpm if you want to use mock.

Building RPMs is a platform specific thing indeed, and the /opt way is a very common way to go (docker-rpm-builder itself is installed in /opt in my prebuilt packages), but you still want to compile in an enviroment as close as the target to be sure your software works.