Our public derivative Docker images should be OSS

Created by: slimsag

Lots of these should be open-source, but are currently locked behind our private infrastructure repo (not intentionally):

• sourcegraph/postgres
• sourcegraph/postgres-11.1
• sourcegraph/redis-cache
• sourcegraph/redis-store
• sourcegraph/redis_exporter
• sourcegraph/zoekt-indexserver
• sourcegraph/zoekt-webserver
• sourcegraph/pgsql-exporter
• sourcegraph/pod-tmp-gc

And perhaps many more. Of course, we should do some cleanup at the same time:

• pgsql-exporter vs redis_exporter vs redis-cache naming inconsistency.
• why is postgres-11.1 a different name and not just a tag?