[deploy-sourcegraph] re-add redis-store pure docker simplifications

[ggilmore]

See https://github.com/sourcegraph/deploy-sourcegraph

https://github.com/sourcegraph/sourcegraph/issues/1761#issuecomment-453288013

[emidoots]

Very bad that we're not using this in deploy-sourcegraph, so tagging for 3.1. But not critical for 3.0 and I won't have time to complete it.