docs(core): clarify developing server docs (#9420)

.docker/dev/compose.yml.example

@@ -11,7 +11,7 @@ services:
       POSTGRES_USER: ${DB_USERNAME}
       POSTGRES_DB: ${DB_DATABASE_NAME}
     volumes:
-      - ${DATABASE_LOCATION}:/var/lib/postgresql/data
+      - postgres_data:/var/lib/postgresql/data
 
   redis:
     image: redis:latest
@@ -26,3 +26,6 @@ services:
 
 networks:
   dev:
+
+volumes:
+  postgres_data: