fix/hive-pgsql: Fix Postgres volume mount and healthcheck in Hive DB service

Update volume mount to /var/lib/postgresql to avoid nested data dir issues
introduced in postgres:18-alpine and later (docker-library/postgres#1394),
and use $$POSTGRES_USER env var in healthcheck for consistency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: iobruno

module5-batch-processing/compose.spark-4.1-standalone.yaml

@@ -109,9 +109,9 @@ services:
     ports:
       - '5433:5432'
     volumes:
-      - vol-hive-db:/var/lib/postgresql/data
+      - vol-hive-db:/var/lib/postgresql
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      test: ["CMD-SHELL", "pg_isready -U $$POSTGRES_USER"]
       interval: 5s
       timeout: 5s
       retries: 5