Add kamal-proxy in experimental mode

The proxy can be enabled via the config:

```
proxy:
  enabled: true
  hosts:
    - 10.0.0.1
    - 10.0.0.2
```

This will enable the proxy and cause it to be run on the hosts listed
under `hosts`, after running `kamal proxy reboot`.

Enabling the proxy disables `kamal traefik` commands and replaces them
with `kamal proxy` ones. However only the marked hosts will run the
kamal-proxy container, the rest will run Traefik as before.

test/commands/proxy_test.rb

@@ -0,0 +1,126 @@
+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 kamal-proxy --detach --restart unless-stopped --publish 80:80 --publish 443:443 --volume /var/run/docker.sock:/var/run/docker.sock --volume kamal-proxy:/root/.config/kamal-proxy --log-opt max-size=\"10m\" #{Kamal::Configuration::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "run with ports configured" do
+    assert_equal \
+      "docker run --name kamal-proxy --detach --restart unless-stopped --publish 80:80 --publish 443:443 --volume /var/run/docker.sock:/var/run/docker.sock --volume kamal-proxy:/root/.config/kamal-proxy --log-opt max-size=\"10m\" #{Kamal::Configuration::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "run without configuration" do
+    @config.delete(:proxy)
+
+    assert_equal \
+      "docker run --name kamal-proxy --detach --restart unless-stopped --publish 80:80 --publish 443:443 --volume /var/run/docker.sock:/var/run/docker.sock --volume kamal-proxy:/root/.config/kamal-proxy --log-opt max-size=\"10m\" #{Kamal::Configuration::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 kamal-proxy --detach --restart unless-stopped --publish 80:80 --publish 443:443 --volume /var/run/docker.sock:/var/run/docker.sock --volume kamal-proxy:/root/.config/kamal-proxy --log-driver \"local\" --log-opt max-size=\"100m\" --log-opt max-file=\"3\" #{Kamal::Configuration::Proxy::DEFAULT_IMAGE}",
+      new_command.run.join(" ")
+  end
+
+  test "proxy start" do
+    assert_equal \
+      "docker container start kamal-proxy",
+      new_command.start.join(" ")
+  end
+
+  test "proxy stop" do
+    assert_equal \
+      "docker container stop kamal-proxy",
+      new_command.stop.join(" ")
+  end
+
+  test "proxy info" do
+    assert_equal \
+      "docker ps --filter name=^kamal-proxy$",
+      new_command.info.join(" ")
+  end
+
+  test "proxy logs" do
+    assert_equal \
+      "docker logs kamal-proxy --timestamps 2>&1",
+      new_command.logs.join(" ")
+  end
+
+  test "proxy logs since 2h" do
+    assert_equal \
+      "docker logs kamal-proxy  --since 2h --timestamps 2>&1",
+      new_command.logs(since: "2h").join(" ")
+  end
+
+  test "proxy logs last 10 lines" do
+    assert_equal \
+      "docker logs kamal-proxy  --tail 10 --timestamps 2>&1",
+      new_command.logs(lines: 10).join(" ")
+  end
+
+  test "proxy logs with grep hello!" do
+    assert_equal \
+      "docker logs kamal-proxy --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=kamal-proxy",
+      new_command.remove_container.join(" ")
+  end
+
+  test "proxy remove image" do
+    assert_equal \
+      "docker image prune --all --force --filter label=org.opencontainers.image.title=kamal-proxy",
+      new_command.remove_image.join(" ")
+  end
+
+  test "proxy follow logs" do
+    assert_equal \
+      "ssh -t root@1.1.1.1 -p 22 'docker logs kamal-proxy --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 kamal-proxy --timestamps --tail 10 --follow 2>&1 | grep \"hello!\"'",
+      new_command.follow_logs(host: @config[:servers].first, grep: "hello!")
+  end
+
+  test "deploy" do
+    assert_equal \
+      "docker exec kamal-proxy kamal-proxy deploy service --target \"172.1.0.2:80\"",
+      new_command.deploy("service", target: "172.1.0.2:80").join(" ")
+  end
+
+  test "remove" do
+    assert_equal \
+      "docker exec kamal-proxy kamal-proxy remove service --target \"172.1.0.2:80\"",
+      new_command.remove("service", target: "172.1.0.2:80").join(" ")
+  end
+
+  private
+    def new_command
+      Kamal::Commands::Proxy.new(Kamal::Configuration.new(@config, version: "123"))
+    end
+end