updated .env insertion for db to use new .env variables

compose.yaml

@@ -16,9 +16,9 @@ services:
   db:
     image: postgres:18
     environment:
-      POSTGRES_DB: ${DB_NAME}
-      POSTGRES_USER: ${DB_USER}
-      POSTGRES_PASSWORD: ${DB_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
     volumes:
       - postgres_data:/var/lib/postgresql
     restart: unless-stopped