Faster CI feedback with build against standard image

Right now we put a lot more stress on the docker build server than potentially necessary because we start a docker build of eic_container/containers/jug/xl.Dockerfile to test if juggler builds (and this first builds eicd and npdet).

We may be able to gain from running a preliminary build in image: eicweb.phy.anl.gov:4567/eic/juggler/juggler:$JUGGLER_TAG first before moving on to a docker build.

@sly2j thoughts?