Set an explicit username/password for Postgres in the swarm and k8s files too

docker-stack-simple.yml

@@ -16,6 +16,9 @@ services:
         condition: on-failure
   db:
     image: postgres:9.4
+    environment:
+      POSTGRES_USER: "postgres"
+      POSTGRES_PASSWORD: "postgres"
     volumes:
       - db-data:/var/lib/postgresql/data
     networks:
@@ -78,4 +81,4 @@ networks:
   backend:
 
 volumes:
-  db-data:
+  db-data: