fix(docker): update environment variable for PostgreSQL username in db.yaml

- Changed environment variable from DATABASE_USER to DATABASE_USERNAME for consistency with naming conventions.
- Updated healthcheck command to reflect the new environment variable name.

Author: davidarny

.docker/compose/db.yaml

@@ -12,10 +12,10 @@ services:
     volumes:
       - db-data:/var/lib/postgresql/data
     environment:
-      - DATABASE_USER=${DATABASE_USER:?DATABASE_USER is required}
+      - DATABASE_USERNAME=${DATABASE_USERNAME:?DATABASE_USERNAME is required}
       - DATABASE_PASSWORD=${DATABASE_PASSWORD:?DATABASE_PASSWORD is required}
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U $${DATABASE_USER}"]
+      test: ["CMD-SHELL", "pg_isready -U $${DATABASE_USERNAME}"]
       interval: 10s
       timeout: 5s
       retries: 5