Enable docker buildkit so that sticky bits (for sudo, etc) are preserved
in the container image. They are dropped otherwise. This also requires
docker-ce, which the script will now alert you of if missing in your
environment.
Also remove override placeholders so they don't show up as patchset
changes after running image-builder in a local environment.
Change-Id: I80dfeb12ad6e8ebef71d1f7cf44ad39579ce0d6d
f6dfe38820