Postgres: changed image to the :alpine

All: Check if nginx config exists before updating. This is to prevent overwriting of certbot-auto certificates and redirect

postgres/vars/default.yml

@@ -1,6 +1,6 @@
 ---
 postgres_name: postgres
-postgres_image: postgres
+postgres_image: postgres:alpine
 pg_user: postgres
 pg_password: '123456'
 data_location: /var/lib/postgresql/data