For easy updates from Traefik to kamal-proxy, add `kamal proxy update`.
This stops and removes Traefik and kamal-proxy containers (just in case
it is run after an update). Then it starts kamal-proxy and calls
`kamal-proxy deploy` to route to any app containers that are already
running.
It can be run with a rolling option and calls the `pre-proxy-reboot` and
`post-proxy-reboot` hooks for each host.
[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
