[mproxy](https://github.com/kevinmcconnell/mproxy) 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.
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