Differently than every single other bit that's part of our repo, QEMU
has been using a single Dockerfile that prepares an environment where
the project can be built, but *also* building the project as part of
that very same Dockerfile.
This is a problem, for several different reasons, including:
* It's very hard to have a reproducible build if you don't have an
archived image of the builder
* One cannot cache / ipload the image of the builder, as that contains
already a specific version of QEMU
* Every single CI run we end up building the builder image, which
includes building dependencies (such as liburing)
Let's split the logic into a new build script, and pass the build script
to be executed inside the builder image, which will be only responsible
for providing an environment where QEMU can be built.
Fixes: #5464
Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>
