Update to the latest stable Debian. While we are at it flatten into a
single dockerfile. We really don't need the rest of the stuff from
the QEMU base image just to compile test images.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220914155950.804707-9-alex.bennee@linaro.org>