docker: Add test runner

It's better to have a launcher for all tests, to make it easier to
initialize and manage the environment.

If "DEBUG=1"  a shell prompt will show up before the test runs.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1464755128-32490-7-git-send-email-famz@redhat.com
