Add a writable proxy volume

Maps in and external /home/kamal-proxy/.app-config volume that we can
use to map files to the proxy.

Can be used to store custom maintenance pages or SSL certificates.

lib/kamal/commands/proxy.rb

@@ -109,6 +109,7 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
         "--network", "kamal",
         "--detach",
         "--restart", "unless-stopped",
-        "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy"
+        "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy",
+        *config.proxy_app_config_volume.docker_args
     end
 end

lib/kamal/configuration.rb

@@ -300,6 +300,16 @@ class Kamal::Configuration
     File.join proxy_directory, "image_version"
   end
 
+  def proxy_app_config_directory
+    File.join proxy_directory, "app-config"
+  end
+
+  def proxy_app_config_volume
+    Volume.new \
+      host_path: proxy_app_config_directory,
+      container_path: "/home/kamal-proxy/.app-config"
+  end
+
   def to_h
     {
       roles: role_names,