Buildroot requires x86_64-v3

[supakeen]

So ELN, which is our buildroot, is compiled for x86_64-v3 (see here). People are running into issues regularly where they're running bootc-image-builder on older Xeon hardware that do not support this level of the architecture.

It's probably a good idea to error out much earlier if the sublevel is not supported.

We can use the output of ld.so --help for this.

[cgwalters]

This also touches on #18 because if our very first step is to fetch the target image and execute it (say running true as a sanity check), we will fail at least pretty early on.

IOW we don't need a "buildroot" if the target container is the installer, which compresses complexity here.

Now...there's also the whole larger question of which base image this project uses. I would say that from now, we should use UBI9. But going forward we'll almost certainly need to support being based on multiple distribution/OS bases too (a lot of things here around tie-in to the build process of different OSes/distributions).

[cgwalters]

I think this is fixed by #138