If you are using the UI to create a container, then in the Command tab, there is an option for Command and Entry point. You can try putting it in there. You would also need to put your custom rc.local file in the container, either by building your own custom container, or mounting it in as a volume.
Keep in mind, that the entrypoint/cmd needs to be program or script that runs in the foreground. If your rc.local script starts other programs in the background and then exits, you will not get the desired behavior because once the rc.local script completes, the container will end. This will cause Rancher to restart it, and it will loop.
If your goal is to run several other programs in your container, then you would be better off installing Supervisord (or a similar process manager) and having it manage your other programs. That way, Supervisord runs in the foreground of the container, and your other programs run in the background.