Tag successful builds with "master".
    
    We were looking at deploying the "latest" tag to open.paas/waiverdb-stg, but we
    didn't want to conflate "latest" usable with "latest" passed CI.
    
    The attempt here is to create an automatic tracking tag for everything that has
    merged into master and passed CI, called `master`.  We could have our openshift
    staging environments pull from here while we instead have our openshift prod
    environments pull from a manually curated `latest` tag.
    
    How does that sound?  :)