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/integration/broken_deploy_test.rb

@@ -29,6 +29,5 @@ class BrokenDeployTest < IntegrationTest
       assert_match /First web container is unhealthy on vm[12], not booting any other roles/, output
       assert_match "First web container is unhealthy, not booting workers on vm3", output
       assert_match "nginx: [emerg] unexpected end of file, expecting \";\" or \"}\" in /etc/nginx/conf.d/default.conf:2", output
-      assert_match 'ERROR {"Status":"unhealthy","FailingStreak":0,"Log":[]}', output
     end
 end

test/integration/docker/deployer/app/Dockerfile

@@ -6,4 +6,4 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
-
+RUN echo "Up!" > /usr/share/nginx/html/up

test/integration/docker/deployer/app_with_roles/Dockerfile

@@ -6,4 +6,4 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
-
+RUN echo "Up!" > /usr/share/nginx/html/up

test/integration/docker/deployer/app_with_roles/config/deploy.yml

@@ -9,6 +9,11 @@ servers:
     hosts:
       - vm3
     cmd: sleep infinity
+proxy:
+  enabled: true
+  hosts:
+    - vm2
+  deploy_timeout: 2s
 
 asset_path: /usr/share/nginx/html/versions
 

test/integration/integration_test.rb

@@ -31,7 +31,7 @@ class IntegrationTest < ActiveSupport::TestCase
         succeeded = system("cd test/integration && #{command}")
       end
 
-      raise "Command `#{command}` failed with error code `#{$?}`" if !succeeded && raise_on_error
+      raise "Command `#{command}` failed with error code `#{$?}`, and output:\n#{result}" if !succeeded && raise_on_error
       result
     end
 

test/integration/proxy_test.rb

@@ -0,0 +1,84 @@
+require_relative "integration_test"
+
+class ProxyTest < IntegrationTest
+  setup do
+    @app = "app_with_roles"
+  end
+
+  test "boot, reboot, stop, start, restart, logs, remove" do
+    kamal :envify
+
+    kamal :proxy, :boot
+    assert_proxy_running
+
+    output = kamal :proxy, :reboot, "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-traefik-reboot", "post-traefik-reboot"
+    assert_match /Rebooting Traefik on vm1,vm2.../, output
+    assert_match /Rebooted Traefik on vm1,vm2/, output
+
+    output = kamal :proxy, :reboot, "--rolling", "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-traefik-reboot", "post-traefik-reboot"
+    assert_match /Rebooting Traefik on vm1.../, output
+    assert_match /Rebooted Traefik on vm1/, output
+    assert_match /Rebooting Traefik on vm2.../, output
+    assert_match /Rebooted Traefik on vm2/, output
+
+    kamal :proxy, :boot
+    assert_proxy_running
+    assert_traefik_running
+
+    # Check booting when booted doesn't raise an error
+    kamal :proxy, :stop
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    # Check booting when stopped works
+    kamal :proxy, :boot
+    assert_proxy_running
+    assert_traefik_running
+
+    kamal :proxy, :stop
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    kamal :proxy, :start
+    assert_proxy_running
+    assert_traefik_running
+
+    kamal :proxy, :restart
+    assert_proxy_running
+    assert_traefik_running
+
+    logs = kamal :proxy, :logs, capture: true
+    assert_match /Traefik version [\d.]+ built on/, logs
+
+    kamal :proxy, :remove
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    kamal :env, :delete
+  end
+
+  private
+    def assert_proxy_running
+      assert_match /basecamp\/kamal-proxy:latest   \"kamal-proxy run\"/, proxy_details
+    end
+
+    def assert_proxy_not_running
+      assert_no_match /basecamp\/kamal-proxy:latest   \"kamal-proxy run\"/, proxy_details
+    end
+
+    def assert_traefik_running
+      assert_match /traefik:v2.10   "\/entrypoint.sh/, proxy_details
+    end
+
+    def assert_traefik_not_running
+      assert_no_match /traefik:v2.10   "\/entrypoint.sh/, proxy_details
+    end
+
+    def proxy_details
+      kamal :proxy, :details, capture: true
+    end
+end