Dockerインストールでローカルプラグインディレクトリを使用することは可能ですか?

Call me stupid but how do I change the default port 3000 to something else? d/boot-dev --init fails as I’m using that port for something else. I tried -e UNICORN_PORT=4200. The guides I’ve seen say nothing about the port. The thin.yml file in config/cloud/cloud66/files seems to be ignored too