deploy-sourcegraph-docker: some regressions in master from new prometheus/grafana setup

Created by: slimsag

1. The Prometheus version changed from v1.4.1 to v2.12.0 but those versions are not backwards compatible, so any user upgrading would find Prometheus unable to start I believe.
2. The docker.rules file was removed, and isn't included by default in the Docker image. This will break the containers dashboard that is built-in. https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/39/files#diff-da9b899ce79b0173475a4afcb8ae376b (also, that containers dashboard probably will not work in other deploy contexts currently, since it relies on metrics that only exist in deploy-sourcegraph-docker -- we'll need to define some common rules across all contexts to fix this)

@uwedeportivo I will fix #1 (closed) above, do you know what I would need to set to load the docker.rules just in the deploy-sourcegraph-docker context?