Add support for configuration extensions

Allow blocks prefixed with `x-` in the configuration as a place to
declare reusable blocks with YAML anchors and aliases.

Borrowed from the Docker Compose configuration file format -
https://github.com/compose-spec/compose-spec/blob/main/spec.md#extension

Thanks to @ruyrocha for the suggestion.

lib/kamal/configuration.rb

@@ -47,7 +47,7 @@ class Kamal::Configuration
     @destination = destination
     @declared_version = version
 
-    validate! raw_config, example: validation_yml.symbolize_keys, context: ""
+    validate! raw_config, example: validation_yml.symbolize_keys, context: "", with: Kamal::Configuration::Validator::Configuration
 
     # Eager load config to validate it, these are first as they have dependencies later on
     @servers = Servers.new(config: self)

lib/kamal/configuration/docs/configuration.yml

@@ -2,13 +2,24 @@
 #
 # Configuration is read from the `config/deploy.yml`
 #
+
+# Destinations
+#
 # When running commands, you can specify a destination with the `-d` flag,
 # e.g. `kamal deploy -d staging`
 #
 # In this case the configuration will also be read from `config/deploy.staging.yml`
 # and merged with the base configuration.
+
+# Extensions
 #
-# The available configuration options are explained below.
+# Kamal will not accept unrecognized keys in the configuration file.
+#
+# However, you might want to declare a configuration block using YAML anchors
+# and aliases to avoid repetition.
+#
+# You can use prefix a configuration section with `x-` to indicate that it is an
+# extension. Kamal will ignore the extension and not raise an error.
 
 # The service name
 # This is a required value. It is used as the container name prefix.

lib/kamal/configuration/validator.rb

@@ -15,11 +15,10 @@ class Kamal::Configuration::Validator
     def validate_against_example!(validation_config, example)
       validate_type! validation_config, Hash
 
-      if (unknown_keys = validation_config.keys - example.keys).any?
-        unknown_keys_error unknown_keys
-      end
+      check_unknown_keys! validation_config, example
 
       validation_config.each do |key, value|
+        next if extension?(key)
         with_context(key) do
           example_value = example[key]
 
@@ -137,4 +136,18 @@ class Kamal::Configuration::Validator
     ensure
       @context = old_context
     end
+
+    def allow_extensions?
+      false
+    end
+
+    def extension?(key)
+      key.to_s.start_with?("x-")
+    end
+
+    def check_unknown_keys!(config, example)
+      unknown_keys = config.keys - example.keys
+      unknown_keys.reject! { |key| extension?(key) } if allow_extensions?
+      unknown_keys_error unknown_keys if unknown_keys.present?
+    end
 end

lib/kamal/configuration/validator/configuration.rb

@@ -0,0 +1,6 @@
+class Kamal::Configuration::Validator::Configuration < Kamal::Configuration::Validator
+  private
+    def allow_extensions?
+      true
+    end
+end