codeintel: RFC 235: Add code intel postgres image
Created by: efritz
A step towards RFC 235: Move LSIF data to Postgres. Closes https://github.com/sourcegraph/sourcegraph/issues/13912.
This creates a new image that can be used as the code intel Postgres database. We're giving it a name instead of using the same postgres container in order to have a more consistent and less implementation-specific name.