Uniquely identify builds to prevent race

In the original implementation of this, the latest running container
containing "PICKME" was selected for the container to export. This
creates a race condition.

Instead we now identify the image by a UUID created each run, so that
they will not overlap if multiple builds happen simultaneously.

Change-Id: Iaaf5683435f424517774067a1d95732ad096c988
Merge "Uniquely identify builds to prevent race"