Add PostgreSQL dir with socket to Docker container as a volume

Also explicitly set path to pg_dump 9.6 to avoid using old 9.5 postgres
client.

Fixes: PROD-31827

Change-Id: I727073c1caec40f11b87afcd64e31cdc50185a83
(cherry-picked from commit a1b31b7f2f39cfcd0ee2856a10a96f35d4c5e80c)
diff --git a/docker/swarm/stack/postgresql.yml b/docker/swarm/stack/postgresql.yml
index 12eb874..2d6d7e7 100644
--- a/docker/swarm/stack/postgresql.yml
+++ b/docker/swarm/stack/postgresql.yml
@@ -26,5 +26,6 @@
                   condition: any
               volumes:
                 - /srv/volumes/postgresql/data:/var/lib/postgresql/data
+                - /var/run/postgresql:/var/run/postgresql
               ports:
                 - ${_param:postgresql_exposed_port}:${_param:postgresql_bind_port}