Require an arch to be set, and default to amd64 in the template

lib/kamal/cli/templates/deploy.yml

@@ -18,6 +18,10 @@ registry:
   password:
     - KAMAL_REGISTRY_PASSWORD
 
+# Configure builder setup.
+builder:
+  arch: amd64
+
 # Inject ENV variables into containers (secrets come from .env).
 # Remember to run `kamal env push` after making changes!
 # env:
@@ -30,16 +34,6 @@ registry:
 # ssh:
 #   user: app
 
-# Configure builder setup.
-# builder:
-#   args:
-#     RUBY_VERSION: 3.2.0
-#   secrets:
-#     - GITHUB_TOKEN
-#   remote:
-#     arch: amd64
-#     host: ssh://app@192.168.0.1
-
 # Use accessory services (secrets come from .env).
 # accessories:
 #   db:

lib/kamal/configuration/validator/builder.rb

@@ -5,5 +5,7 @@ class Kamal::Configuration::Validator::Builder < Kamal::Configuration::Validator
     if config["cache"] && config["cache"]["type"]
       error "Invalid cache type: #{config["cache"]["type"]}" unless [ "gha", "registry" ].include?(config["cache"]["type"])
     end
+
+    error "Builder arch not set" unless config["arch"].present?
   end
 end