* style: Rename TASK to STEP
* do not pull base image after it was copied from node0 and loaded.
It caused the current Dockerfile to be out-of-sync with the current image.
* do not pull images after loading the downloaded and up-to-date
base.tar. It was overwriting the base.tar downloaded and loaded.
* save debian:jessie into base.tar
* use "time" command to help identify long running operation
* rename marker file from "BASE_READY" to "BASE_AVAILABLE"
* build base image only if needed.
