Add KAMAL_HOST to app and accessory containers

Adds the host the container is being deployed to as KAMAL_HOST.
My use case is to more easily tag the host for metrics tagging,
but there might be other uses as well.

lib/kamal/cli/accessory.rb

@@ -13,11 +13,11 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
           directories(name)
           upload(name)
 
-          on(hosts) do
+          on(hosts) do |host|
             execute *KAMAL.auditor.record("Booted #{name} accessory"), verbosity: :debug
             execute *accessory.ensure_env_directory
             upload! accessory.secrets_io, accessory.secrets_path, mode: "0600"
-            execute *accessory.run
+            execute *accessory.run(host: host)
 
             if accessory.running_proxy?
               target = capture_with_info(*accessory.container_id_for(container_name: accessory.service_name, only_running: true)).strip

lib/kamal/commands/accessory.rb

@@ -13,11 +13,12 @@ class Kamal::Commands::Accessory < Kamal::Commands::Base
     @accessory_config = config.accessory(name)
   end
 
-  def run
+  def run(host: nil)
     docker :run,
       "--name", service_name,
       "--detach",
       "--restart", "unless-stopped",
+      *([ "-e", "KAMAL_HOST=\"#{host}\"" ] if host),
       *network_args,
       *config.logging_args,
       *publish_args,

lib/kamal/commands/app.rb

@@ -22,6 +22,7 @@ class Kamal::Commands::App < Kamal::Commands::Base
       *([ "--hostname", hostname ] if hostname),
       "-e", "KAMAL_CONTAINER_NAME=\"#{container_name}\"",
       "-e", "KAMAL_VERSION=\"#{config.version}\"",
+      "-e", "KAMAL_HOST=\"#{host}\"",
       *role.env_args(host),
       *role.logging_args,
       *config.volume_args,