I managed to create a working shell with the following runner:
FROM codenvy/shellinabox
RUN sudo apt-get update && sudo apt-get -y install g++ git htop ssh tmux vim python
VOLUME ["/home/user/app"]
ENV CODENVY_APP_BIND_DIR /home/user/app
CMD sleep 365d
This will sync changes from the Codenvy WebIDE to the terminal.
But it won't sync changes from the terminal to the Codenvy WebIDE(even if you attempt to refresh,close/open file, etc).