Document how to build a disk image and testing with a custom package

[alicefr]

In the case, we have a bug in any of the node components, we need to be able to replace the package/binary with a custom one and test eventual fix. An example could be if there is a bug in bootc and we want to verify if a patch solved the problem.

[alicefr]

cc @HarshwardhanPatil07

[jlebon]

We should definitely also document the case where you don't need your custom bootc/$pkg in the bootimage itself. In that case, it's much easier:

• build your custom bootc image (i.e. FROM the regular node image, do $whatever you want, e.g. dnf install stuff, or use make && make install ... in a multi-stage build, etc...) and push it
• point the bootcnodepool to your built image and let it roll it out to your worker(s) for you

That said, I think documenting the custom bootimage flow absolutely still does make sense because sometimes what you're testing just needs to be there from the very start.