fix(docker-compose): Remove the "data" directory from the volume mount

See the comment at [1]. This fixes the startup error saying:

```
In 18+, these Docker images are configured to store database data in a
format which is compatible with "pg_ctlcluster" (specifically, using
major-version-specific directory names).  This better reflects how
PostgreSQL itself works, and how upgrades are to be performed.

See also docker-library/postgres#1259

Counter to that, there appears to be PostgreSQL data in:
  /var/lib/postgresql/data (unused mount/volume)

This is usually the result of upgrading the Docker image without
upgrading the underlying database using "pg_upgrade" (which requires both
versions).

The suggested container configuration for 18+ is to place a single mount
at /var/lib/postgresql which will then place PostgreSQL data in a
subdirectory, allowing usage of "pg_upgrade --link" without mount point
boundary issues.
```

[1]: docker-library/postgres#1259 (comment)

Signed-off-by: Sebastian Schuberth <[email protected]>

Author: sschuberth

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     db:
         image: postgres:18.0
         volumes:
-            - db-data:/var/lib/postgresql/data
+            - db-data:/var/lib/postgresql
         ports:
             - 5432:5432
         environment: