Hi Gábor,
The "htdocs" warning can be safely ignored. That's just a deprecation warning that we issue for older extensions. Current Power Pack releases still bundle a legacy way of shipping static media files for wide Review Board compatibilty, hence that warning. It won't actually impact anything on the system.
The important thing to note when using Docker is that Review Board has two halves:
1. The backend server, which serves up the Review Board API and URLs, and is meant to be accessed directly only by the frontend server.
2. The frontend server, which serves up CSS, JavaScript, and images, and sends all other requests to the backend server.
In the example Docker configuration, that port 8080 is for Review Board's backend server, so it's expected that you won't get the media files. You'll need to access the frontend server instead. In our example setup, that's Nginx, port 80.
Christian