fix/prefect-pgsql-mount: Fix Postgres volume mount and healthcheck

Fix volume mount point from /var/lib/postgresql/data to /var/lib/postgresql
and use $$POSTGRES_USER env var in pg_isready healthcheck for both
ingest-db and prefect-metastore services.

This addresses a breaking change in postgres:18-alpine and later versions
introduced by docker-library/postgres#1394.

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

Author: iobruno

module2-workflow-orchestration/prefect/compose.yaml

@@ -12,9 +12,9 @@ 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 $$POSTGRES_USER"]
       interval: 5s
       timeout: 5s
       retries: 5
@@ -30,9 +30,9 @@ services:
     ports:
       - '5432'
     volumes:
-      - vol-prefect-metastore:/var/lib/postgresql/data
+      - vol-prefect-metastore:/var/lib/postgresql
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U prefect"]
+      test: ["CMD-SHELL", "pg_isready -U $$POSTGRES_USER"]
       interval: 5s
       timeout: 5s
       retries: 5