tests/docker/docker.py: docker_dir outside build

tests/docker/docker.py: docker_dir outside build

Instead of letting the build_image create the temporary working dir we
move the creation to the build command. This is preparation for the
later patches where additional files can be added to the build context
before the build step is run.

We also ensure we remove the build context after we are done (mkdtemp
doesn't do this automatically for you).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1468934445-32183-2-git-send-email-famz@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>