Tidy up postgres compose

compose.postgres.yml

@@ -2,20 +2,7 @@
 version: '3.8'
 
 services:
-  db:
-    image: "postgres:9.6.5"
-    volumes:
-      - "db:/var/lib/postgresql/data"
-    networks:
-      - internal
-    environment:
-      POSTGRES_USER: selfoss
-      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
-      POSTGRES_DB: selfoss
-    secrets:
-      - db_password
-
-  selfoss: 
+  app: 
     networks:
       - internal
     depends_on:
@@ -28,8 +15,21 @@ services:
       - SELFOSS_DB_USERNAME=selfoss
       - SELFOSS_DB_PASSWORD_FILE=/run/secrets/db_password
 
-volumes:
   db:
+    image: "postgres:9.6.5"
+    volumes:
+      - "postgres:/var/lib/postgresql/data"
+    networks:
+      - internal
+    environment:
+      POSTGRES_USER: selfoss
+      POSTGRES_PASSWORD_FILE: /run/secrets/db_password
+      POSTGRES_DB: selfoss
+    secrets:
+      - db_password
+
+volumes:
+  postgres:
 
 secrets:
   db_password: