tests/docker: make "buildah bud" output similar to "docker build"

Podman users will most often be using buildah to build containers.
Among the differences between "buildah bud|build-using-dockerfile" and
a traditional "docker build" is that buildah does not run a container
during build.

To the best of my knowledge and experiments, this means that runtime
variables, such as ENV from one base image will not propagate into
another.  The end result is that the location for the cross compiler
binaries, defined in the base "qemu/debian9-mxe" image, are not passed
through this image.  Consequently, the cross compilers are not on PATH
and the build fails.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200312193616.438922-3-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>