Use staging registry #44

This merge request removes the assignment of the BASE_URL env variable inside of wl_compare_lint.py (since we already have a CI variable for that) and adds logic to build-run.sh script to determine what registry to assign as the BASE_REGISTRY based on the branch the pipeline is running on. Additionally, images pushed to the staging registry will be tagged twice (once with the build number appended and once without). This is being done to address issue 44, which requests that images in feature and dev branches are able to be built from unapproved containers that aren't in prod yet. Master branch pipelines will still pull their base images from prod, preventing any unapproved base layers from being pushed to prod.