Oh! Within the container? Interesting! This means that your flask server is not running (crashed) or that you are using a flask config that has overwritten the default port 5000 (may be 8080?). In case of a crash, may be try to attach to the running container and investigate that. Then try running the flask server yourself to see the reason of the crash. Otherwise investigate the appropriate internal port your server is running off and that may work for the mapping.