fix/dbt-postgres: Fix postgres volume mount and healthcheck

Fix for postgres:18-alpine and later versions, since the default data
directory changed from /var/lib/postgresql/data to /var/lib/postgresql.
See: docker-library/postgres#1394

Also use $$POSTGRES_USER env var in healthcheck instead of hardcoded username.

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

Author: iobruno

module4-analytics-engineering/postgres/compose.yaml

@@ -11,9 +11,9 @@ services:
     ports:
       - '5432:5432'
     volumes:
-      - vol-ingest-db:/var/lib/postgresql/data
+      - vol-ingest-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