Split the creation of the test images and the creation of the
docker environment. This way, one can easily recreate the test
images, without requiring to re-provision everything.
Change-Id: I35bbe8d8bf497f16d9be92cbaa2d9001f8eee247
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>
Reviewed-by: Ryan Chu <ryan.chu@qt.io>