Fix example postgres docker-compose setup in docs (#1998)

* Fix example postgres docker-compose setup in docs
Add a local volume to postgres container so changes get persisted between restarts

* Fix linked volume in postgres doc

docs/docs/documentation/getting-started/installation/postgres.md

@@ -55,6 +55,8 @@ services:
     container_name: postgres
     image: postgres
     restart: always
+    volumes:
+      - mealie-pgdata:/var/lib/postgresql
     environment:
       POSTGRES_PASSWORD: mealie
       POSTGRES_USER: mealie
@@ -62,6 +64,8 @@ services:
 volumes:
   mealie-data:
     driver: local
+  mealie-pgdata:
+    driver: local
 ```
 
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