[kamal-proxy](https://github.com/basecamp/kamal-proxy) is a custom
minimal proxy designed specifically for Kamal.
It has some 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 a proxy without conflicting config.
3. First class support for Kamal operations - rather than trying to
work out how to make Traefik do what we want, we can build features
directly into the proxy, making configuration simpler and avoiding
obscure errors
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.
