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/defaults/postgresql/init.yml b/defaults/postgresql/init.yml
new file mode 100644
index 0000000..7ee5704
--- /dev/null
+++ b/defaults/postgresql/init.yml
@@ -0,0 +1,4 @@
+parameters:
+  _param:
+    postgresql_major_version: '9.6'
+