Using a custom container image, we reduce the test time from 5 minutes to 2 minutes as we no longer run gclient sync from zero.

In the future, we may be able to cache the ninja output in the image (which probably gets updated daily) so we no longer compile from zero.

This reduces the build time from 5 minutes to 3 minutes as we no longer need to cold start a GCE instance. Moreover, we can now compile the host engine using this container.

The whole process is less than 5 minutes using a 24-CPU GCE instance.

I may further improve it by using containers and more CPUs in the following PRs.