Are you sure your app is listening on the same port that is declared using an EXPOSE directive?
Are you sure your app is expecting websocket traffic over HTTP (as opposed to HTTPS?)
Does your image expose multiple ports?
The health check takes a while to fail (10 minutes - we plan to reduce that), but I suggest waiting until it ends; that’ll provide more specific information regarding why it failed. If you did wait, what was the final reported error?