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.