tests/docker/docker.py: check and run .pre script

The docker script will now search for an associated $dockerfile.pre
script which gets run in the same build context as the dockerfile will
be. This is to support pre-seeding the build context before running the
docker build.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1468934445-32183-4-git-send-email-famz@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>
