[mproxy](https://github.com/basecamp/parachute) is a custom minimal
proxy designed specifically for Kamal.
It has two big advantages over Traefik:
1. Imperative deployments - we tell it to switch from container A to
container B, and it waits for container B to start then switches. No
need to poll for health checks ourselves or mess around with forcing
health checks to fail.
2. Support for multiple apps - as much as possible, configuration is
supplied at runtime by the deploy command, allowing us to have
multiple apps share an instance of mproxy without conflicting config.
Add an app with roles to the integration tests. We'll deploy two web
containers and one worker. The worker just sleeps, so we are testing
that the container has booted.
