We want to migrate from docker.py to building our images directly with
docker/podman. Before we get there we need to make sure we don't
re-introduce our layered builds so bug out if we see FROM qemu/ in a
Dockerfile.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-30-alex.bennee@linaro.org>