Map kamal proxy config into .kamal/proxy/config

This will allow us to share files with the proxy via the host.

lib/kamal/commands/proxy.rb

@@ -17,7 +17,7 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
       "--restart", "unless-stopped",
       *proxy_config.publish_args,
       "--volume", "/var/run/docker.sock:/var/run/docker.sock",
-      "--volume", "#{container_name}:/root/.config/kamal-proxy",
+      "--volume", "#{proxy_config.config_directory_as_docker_volume}:/root/.config/kamal-proxy",
       *config.logging_args,
       proxy_config.image
   end

lib/kamal/configuration/proxy.rb

@@ -9,6 +9,7 @@ class Kamal::Configuration::Proxy
   delegate :argumentize, :optionize, to: Kamal::Utils
 
   def initialize(config:)
+    @config = config
     @proxy_config = config.raw_config.proxy || {}
     validate! proxy_config, with: Kamal::Configuration::Validator::Proxy
   end
@@ -70,6 +71,10 @@ class Kamal::Configuration::Proxy
     optionize deploy_options
   end
 
+  def config_directory_as_docker_volume
+    File.join config.run_directory_as_docker_volume, "proxy", "config"
+  end
+
   private
-    attr_accessor :proxy_config
+    attr_reader :config, :proxy_config
 end