Improve README
Improve README

* Added explanation for env vars
* How to run setup in background
* How to run an HTCondor "cluster"
* Using Singularity for dependency resolution
* What is IMAGE_TAG used for?
* "SSH"ing into a container
* Build containers locally
* Adding additional containers or configurations
* Some minor fixes/updates

chambm/docker-galaxy-stable,bgruening/docker-galaxy-stable,bgruening/docker-galaxy-stable,bgruening/docker-galaxy-stable,chambm/docker-galaxy-stable,chambm/docker-galaxy-stable