include Postgres driver in iceberg-rest-fixture image

### Feature Request / Improvement

by default the image use an sqlite db

would be great that the image can directly use a remote postgres without need to create a custom image

because currently doing

CATALOG_URI=jdbc:postgresql://iceberg_rest-db:5432/iceberg_restdb

fail with

Caused by: java.sql.SQLException: No suitable driver found for jdbc:postgresql://iceberg_rest-db:5432/iceberg_restdb

### Query engine

None

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

The issue identifies the iceberg-rest-fixture image and a JDBC PostgreSQL URL as the entry points. Locate the image definition and its dependency setup, then verify that the image can connect to a remote PostgreSQL database without a custom build. Done means the example URL no longer fails with “No suitable driver...”.

Written by the indexing model from the issue text.