Update pure_docker.md
Created by: caugustus-sourcegraph
https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/748 - the initial diff had an incorrect version for pgsql, so we had to force-push to get a single diff.
Created by: caugustus-sourcegraph
https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/748 - the initial diff had an incorrect version for pgsql, so we had to force-push to get a single diff.