RFC 128: Implement consistent Docker image versioning

Created by: slimsag

Per RFC 128 (tracking issue) we should implement consistent versioning of our Docker images so that it is easy for admins to upgrade Sourcegraph and use a version of Sourcegraph released from master (insiders does not currently solve this, see the RFC for details about why.)

1. Push Sourcegraph-versioned images of images that are built in this repository (Grafana and Prometheus).
2. Push Sourcegraph-versioned images of images that are built outside this repository (zoekt-webserver, zoekt-indexserver, syntect_server, redis-store, redis-cache, pgsql, etc.)