Added example how to read the Ruby version from the .ruby-version file.

lib/kamal/cli/templates/deploy.yml

@@ -36,6 +36,9 @@ registry:
 # Configure builder setup.
 builder:
   arch: amd64
+  # Pass in additional build args needed for your Dockerfile.
+  # args:
+  #   RUBY_VERSION: <%= File.read('.ruby-version').strip %>
 
 # Inject ENV variables into containers (secrets come from .kamal/secrets).
 #