Replace Traefik with mproxy

[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.

test/commands/proxy_test.rb

@@ -0,0 +1,114 @@
+require "test_helper"
+
+class CommandsProxyTest < ActiveSupport::TestCase
+  setup do
+    @config = {
+      service: "app", image: "dhh/app", registry: { "username" => "dhh", "password" => "secret" }, servers: [ "1.1.1.1" ],
+    }
+
+    ENV["EXAMPLE_API_KEY"] = "456"
+  end
+
+  teardown do
+    ENV.delete("EXAMPLE_API_KEY")
+  end
+
+  test "run" do
+    assert_equal \
+      "docker run --name mproxy --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-opt max-size=\"10m\" #{Kamal::Commands::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "run with ports configured" do
+    assert_equal \
+      "docker run --name mproxy --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-opt max-size=\"10m\" #{Kamal::Commands::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "run without configuration" do
+    @config.delete(:proxy)
+
+    assert_equal \
+      "docker run --name mproxy --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-opt max-size=\"10m\" #{Kamal::Commands::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "run with logging config" do
+    @config[:logging] = { "driver" => "local", "options" => { "max-size" => "100m", "max-file" => "3" } }
+
+    assert_equal \
+      "docker run --name mproxy --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-driver \"local\" --log-opt max-size=\"100m\" --log-opt max-file=\"3\" #{Kamal::Commands::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "proxy start" do
+    assert_equal \
+      "docker container start mproxy",
+      new_command.start.join(" ")
+  end
+
+  test "proxy stop" do
+    assert_equal \
+      "docker container stop mproxy",
+      new_command.stop.join(" ")
+  end
+
+  test "proxy info" do
+    assert_equal \
+      "docker ps --filter name=^mproxy$",
+      new_command.info.join(" ")
+  end
+
+  test "proxy logs" do
+    assert_equal \
+      "docker logs mproxy --timestamps 2>&1",
+      new_command.logs.join(" ")
+  end
+
+  test "proxy logs since 2h" do
+    assert_equal \
+      "docker logs mproxy  --since 2h --timestamps 2>&1",
+      new_command.logs(since: '2h').join(" ")
+  end
+
+  test "proxy logs last 10 lines" do
+    assert_equal \
+      "docker logs mproxy  --tail 10 --timestamps 2>&1",
+      new_command.logs(lines: 10).join(" ")
+  end
+
+  test "proxy logs with grep hello!" do
+    assert_equal \
+      "docker logs mproxy --timestamps 2>&1 | grep 'hello!'",
+      new_command.logs(grep: 'hello!').join(" ")
+  end
+
+  test "proxy remove container" do
+    assert_equal \
+      "docker container prune --force --filter label=org.opencontainers.image.title=mproxy",
+      new_command.remove_container.join(" ")
+  end
+
+  test "proxy remove image" do
+    assert_equal \
+      "docker image prune --all --force --filter label=org.opencontainers.image.title=mproxy",
+      new_command.remove_image.join(" ")
+  end
+
+  test "proxy follow logs" do
+    assert_equal \
+      "ssh -t root@1.1.1.1 -p 22 'docker logs mproxy --timestamps --tail 10 --follow 2>&1'",
+      new_command.follow_logs(host: @config[:servers].first)
+  end
+
+  test "proxy follow logs with grep hello!" do
+    assert_equal \
+      "ssh -t root@1.1.1.1 -p 22 'docker logs mproxy --timestamps --tail 10 --follow 2>&1 | grep \"hello!\"'",
+      new_command.follow_logs(host: @config[:servers].first, grep: 'hello!')
+  end
+
+  private
+    def new_command
+      Kamal::Commands::Proxy.new(Kamal::Configuration.new(@config, version: "123"))
+    end
+end