make workspace path configurable via WORKSPACE_PATH env var

Defaults to '.' (current directory) if not set, so existing setups
work unchanged. Override with: WORKSPACE_PATH=/my/path docker compose up

docker-compose.yml

@@ -14,7 +14,7 @@ services:
       - "8000:8000"
     volumes:
       - ./server:/app:ro
-      - ./workspace:/workspace
+      - ${WORKSPACE_PATH:-.}:/workspace
       - pip-cache:/root/.cache/pip
     environment:
       - PYTHONUNBUFFERED=1