fix docker deployment

DOCKER.md

@@ -36,7 +36,7 @@ Deploy **dashboard-cpsp-executive** (HQ) with Docker Compose. Same layout as sit
 
 - Docker 20.10+
 - Docker Compose v2+
-- Ports available: **80** (UI), **18001** (direct API), **15433** (Postgres on localhost)
+- Ports available: **80** (UI; set `FRONTEND_PORT=8080` in `.env` if busy), **18001** (direct API), **15433** (Postgres on localhost)
 
 ```bash
 docker --version