Replace Traefik with parachute

[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.

test/cli/prune_test.rb

@@ -18,12 +18,10 @@ class CliPruneTest < CliTestCase
   test "containers" do
     run_command("containers").tap do |output|
       assert_match /docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +6 | while read container_id; do docker rm $container_id; done on 1.1.1.\d/, output
-      assert_match /docker container prune --force --filter label=service=healthcheck-app on 1.1.1.\d/, output
      end
 
     run_command("containers", "--retain", "10").tap do |output|
       assert_match /docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +11 | while read container_id; do docker rm $container_id; done on 1.1.1.\d/, output
-      assert_match /docker container prune --force --filter label=service=healthcheck-app on 1.1.1.\d/, output
     end
 
     assert_raises(RuntimeError, "retain must be at least 1") do