Proxy boot config

Add commands for managing proxy boot config. Since the proxy can be
shared by multiple applications, the configuration doesn't belong in
`config/deploy.yml`.

Instead you can set the config with:

```
Usage:
  kamal proxy boot_config <set|get|clear>

Options:
      [--publish], [--no-publish], [--skip-publish]   # Publish the proxy ports on the host
                                                      # Default: true
      [--http-port=N]                                 # HTTP port to publish on the host
                                                      # Default: 80
      [--https-port=N]                                # HTTPS port to publish on the host
                                                      # Default: 443
      [--docker-options=option=value option2=value2]  # Docker options to pass to the proxy container
```

By default we boot the proxy with `--publish 80:80 --publish 443:443`.

You can stop it from publishing ports, specify different ports and pass
other docker options.

The config is stored in `.kamal/proxy/options` as arguments to be passed
verbatim to docker run.

Where someone wants to set the options in their application they can do
that by calling `kamal proxy boot_config set` in a pre-deploy hook.

There's an example in the integration tests showing how to use this to
front kamal-proxy with Traefik, using an accessory.

lib/kamal/cli/proxy.rb

@@ -21,6 +21,36 @@ class Kamal::Cli::Proxy < Kamal::Cli::Base
     end
   end
 
+  desc "boot_config <set|get|clear>", "Mange kamal-proxy boot configuration"
+  option :publish, type: :boolean, default: true, desc: "Publish the proxy ports on the host"
+  option :http_port, type: :numeric, default: Kamal::Configuration::PROXY_HTTP_PORT, desc: "HTTP port to publish on the host"
+  option :https_port, type: :numeric, default: Kamal::Configuration::PROXY_HTTPS_PORT, desc: "HTTPS port to publish on the host"
+  option :docker_options, type: :array, default: [], desc: "Docker options to pass to the proxy container", banner: "option=value option2=value2"
+  def boot_config(subcommand)
+    case subcommand
+    when "set"
+      boot_options = [
+        *(KAMAL.config.proxy_publish_args(options[:http_port], options[:https_port]) if options[:publish]),
+        *options[:docker_options].map { |option| "--#{option}" }
+      ]
+
+      on(KAMAL.proxy_hosts) do |host|
+        execute(*KAMAL.proxy.ensure_proxy_directory)
+        upload! StringIO.new(boot_options.join(" ")), KAMAL.config.proxy_options_file
+      end
+    when "get"
+      on(KAMAL.proxy_hosts) do |host|
+        puts "Host #{host}: #{capture_with_info(*KAMAL.proxy.get_boot_options)}"
+      end
+    when "reset"
+      on(KAMAL.proxy_hosts) do |host|
+        execute *KAMAL.proxy.reset_boot_options
+      end
+    else
+      raise ArgumentError, "Unknown boot_config subcommand #{subcommand}"
+    end
+  end
+
   desc "reboot", "Reboot proxy on servers (stop container, remove container, start new container)"
   option :rolling, type: :boolean, default: false, desc: "Reboot proxy on hosts in sequence, rather than in parallel"
   option :confirmed, aliases: "-y", type: :boolean, default: false, desc: "Proceed without confirmation question"
@@ -169,6 +199,7 @@ class Kamal::Cli::Proxy < Kamal::Cli::Base
         stop
         remove_container
         remove_image
+        remove_proxy_directory
       end
     end
   end
@@ -193,6 +224,15 @@ class Kamal::Cli::Proxy < Kamal::Cli::Base
     end
   end
 
+  desc "remove_proxy_directory", "Remove the proxy directory from servers", hide: true
+  def remove_proxy_directory
+    with_lock do
+      on(KAMAL.proxy_hosts) do
+        execute *KAMAL.proxy.remove_proxy_directory, raise_on_non_zero_exit: false
+      end
+    end
+  end
+
   private
     def removal_allowed?(force)
       on(KAMAL.proxy_hosts) do |host|

lib/kamal/commands/proxy.rb

@@ -7,9 +7,8 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
       "--network", "kamal",
       "--detach",
       "--restart", "unless-stopped",
-      *config.proxy_publish_args,
       "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy",
-      *config.logging_args,
+      "\$\(#{get_boot_options.join(" ")}\)",
       config.proxy_image
   end
 
@@ -65,6 +64,22 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
       )
   end
 
+  def ensure_proxy_directory
+    make_directory config.proxy_directory
+  end
+
+  def remove_proxy_directory
+    remove_directory config.proxy_directory
+  end
+
+  def get_boot_options
+    combine [ :cat, config.proxy_options_file ], [ :echo, "\"#{config.proxy_options_default.join(" ")}\"" ], by: "||"
+  end
+
+  def reset_boot_options
+    remove_file config.proxy_options_file
+  end
+
   private
     def container_name
       config.proxy_container_name

lib/kamal/configuration.rb

@@ -246,8 +246,12 @@ class Kamal::Configuration
     env_tags.detect { |t| t.name == name.to_s }
   end
 
-  def proxy_publish_args
-    argumentize "--publish", [ "#{PROXY_HTTP_PORT}:#{PROXY_HTTP_PORT}", "#{PROXY_HTTPS_PORT}:#{PROXY_HTTPS_PORT}" ]
+  def proxy_publish_args(http_port, https_port)
+    argumentize "--publish", [ "#{http_port}:#{PROXY_HTTP_PORT}", "#{https_port}:#{PROXY_HTTPS_PORT}" ]
+  end
+
+  def proxy_options_default
+    proxy_publish_args PROXY_HTTP_PORT, PROXY_HTTPS_PORT
   end
 
   def proxy_image
@@ -258,6 +262,14 @@ class Kamal::Configuration
     "kamal-proxy"
   end
 
+  def proxy_directory
+    File.join run_directory, "proxy"
+  end
+
+  def proxy_options_file
+    File.join proxy_directory, "options"
+  end
+
 
   def to_h
     {