Change PostgreSQL data directory to /var/lib/postgresql/

Starting with PostgreSQL 18, the data directory (PGDATA)
of the Docker Image has changed to be version specific.
For PostgreSQL 18 it is /var/lib/postgresql/18/docker/.

This means that to persist data, mounts and volumes
should target the new location: /var/lib/postgresql/.

Fixes DioCrafts/OxiCloud#281

doc/wopi-integration.md

@@ -1039,7 +1039,7 @@ services:
     networks:
       - oxicloud
     volumes:
-      - pg_data:/var/lib/postgresql/data
+      - pg_data:/var/lib/postgresql/
       - ./db/schema.sql:/docker-entrypoint-initdb.d/10-schema.sql
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U postgres"]