docs/installation/with-docker.md Update PostgreSQL volume path for PostgreSQL 18 (#570)

The Docker Compose example currently mounts the PostgreSQL data directory to `/var/lib/postgresql/data`.

With PostgreSQL 18, the official PostgreSQL Docker image expects the volume to be mounted at `/var/lib/postgresql` instead. Mounting `/var/lib/postgresql/data` causes the container to fail during startup because the image detects PostgreSQL data at an unused mount point.

This change updates the example to use the recommended `/var/lib/postgresql` mount path for PostgreSQL 18 and newer.

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/570
Reviewed-by: silverwind <[email protected]>
Co-authored-by: JWheelMaker <[email protected]>

docs/installation/with-docker.md

@@ -191,7 +191,7 @@ services:
 +    networks:
 +      - gitea
 +    volumes:
-+      - ./postgres:/var/lib/postgresql/data
++      - ./postgres:/var/lib/postgresql
 ```
 
 ## Named volumes

versioned_docs/version-28/installation/with-docker.md

@@ -191,7 +191,7 @@ services:
 +    networks:
 +      - gitea
 +    volumes:
-+      - ./postgres:/var/lib/postgresql/data
++      - ./postgres:/var/lib/postgresql
 ```
 
 ## Named volumes