Using docker links isn't a good method for scaling. Instead we will add consul and use ContainerPilot to register our services with consul for us. Once this is done we can use the docker-compose scale command to add more instances of the frontend and have them balanced through the gateway. Your challenge is to update the gateway/etc/containerpilot.json5 file to run the bin/generate-config file before starting the gateway and on any change to the frontend that is detected in consul.
hint ContainerPilot documentation can be found at https://www.joyent.com/containerpilot/docs
hint You need to add a job to execute generate-config