Configuration validation

Validate the Kamal configuration giving useful warning on errors.
Each section of the configuration has its own config class and a YAML
file containing documented example configuration.

You can run `kamal docs` to see the example configuration, and
`kamal docs <section>` to see the example configuration for a specific
section.

The validation matches the configuration to the example configuration
checking that there are no unknown keys and that the values are of
matching types.

Where there is more complex validation - e.g for envs and servers, we
have custom validators that implement those rules.

Additonally the configuration examples are used to generate the
configuration documentation in the kamal-site repo.

You generate them by running:

```
bundle exec bin/docs <kamal-site-checkout>
```

lib/kamal/configuration/builder.rb

@@ -1,73 +1,79 @@
 class Kamal::Configuration::Builder
-  def initialize(config:)
-    @options = config.raw_config.builder || {}
-    @image = config.image
-    @server = config.registry["server"]
-    @service = config.service
-    @destination = config.destination
+  include Kamal::Configuration::Validation
 
-    valid?
+  attr_reader :config, :builder_config
+  delegate :image, :service, to: :config
+  delegate :server, to: :"config.registry"
+
+  def initialize(config:)
+    @config = config
+    @builder_config = config.raw_config.builder || {}
+    @image = config.image
+    @server = config.registry.server
+    @service = config.service
+
+    validate! builder_config, with: Kamal::Configuration::Validator::Builder
   end
 
   def to_h
-    @options
+    builder_config
   end
 
   def multiarch?
-    @options["multiarch"] != false
+    builder_config["multiarch"] != false
   end
 
   def local?
-    !!@options["local"]
+    !!builder_config["local"]
   end
 
   def remote?
-    !!@options["remote"]
+    !!builder_config["remote"]
   end
 
   def cached?
-    !!@options["cache"]
+    !!builder_config["cache"]
   end
 
   def args
-    @options["args"] || {}
+    builder_config["args"] || {}
   end
 
   def secrets
-    @options["secrets"] || []
+    builder_config["secrets"] || []
   end
 
   def dockerfile
-    @options["dockerfile"] || "Dockerfile"
+    builder_config["dockerfile"] || "Dockerfile"
   end
 
   def target
-    @options["target"]
+    builder_config["target"]
   end
 
   def context
-    @options["context"] || "."
+    builder_config["context"] || "."
   end
 
   def local_arch
-    @options["local"]["arch"] if local?
+    builder_config["local"]["arch"] if local?
   end
 
   def local_host
-    @options["local"]["host"] if local?
+    builder_config["local"]["host"] if local?
   end
 
   def remote_arch
-    @options["remote"]["arch"] if remote?
+    builder_config["remote"]["arch"] if remote?
   end
 
   def remote_host
-    @options["remote"]["host"] if remote?
+    builder_config["remote"]["host"] if remote?
   end
 
   def cache_from
     if cached?
-      case @options["cache"]["type"]
+      case builder_config["cache"]["type"]
       when "gha"
         cache_from_config_for_gha
       when "registry"
@@ -78,7 +84,7 @@ class Kamal::Configuration::Builder
 
   def cache_to
     if cached?
-      case @options["cache"]["type"]
+      case builder_config["cache"]["type"]
       when "gha"
         cache_to_config_for_gha
       when "registry"
@@ -88,15 +94,15 @@ class Kamal::Configuration::Builder
   end
 
   def ssh
-    @options["ssh"]
+    builder_config["ssh"]
   end
 
   def git_clone?
-    Kamal::Git.used? && @options["context"].nil?
+    Kamal::Git.used? && builder_config["context"].nil?
   end
 
   def clone_directory
-    @clone_directory ||= File.join Dir.tmpdir, "kamal-clones", [ @service, pwd_sha ].compact.join("-")
+    @clone_directory ||= File.join Dir.tmpdir, "kamal-clones", [ service, pwd_sha ].compact.join("-")
   end
 
   def build_directory
@@ -109,18 +115,12 @@ class Kamal::Configuration::Builder
   end
 
   private
-    def valid?
-      if @options["cache"] && @options["cache"]["type"]
-        raise ArgumentError, "Invalid cache type: #{@options["cache"]["type"]}" unless [ "gha", "registry" ].include?(@options["cache"]["type"])
-      end
-    end
-
     def cache_image
-      @options["cache"]&.fetch("image", nil) || "#{@image}-build-cache"
+      builder_config["cache"]&.fetch("image", nil) || "#{image}-build-cache"
     end
 
     def cache_image_ref
-      [ @server, cache_image ].compact.join("/")
+      [ server, cache_image ].compact.join("/")
     end
 
     def cache_from_config_for_gha
@@ -132,11 +132,11 @@ class Kamal::Configuration::Builder
     end
 
     def cache_to_config_for_gha
-      [ "type=gha", @options["cache"]&.fetch("options", nil) ].compact.join(",")
+      [ "type=gha", builder_config["cache"]&.fetch("options", nil) ].compact.join(",")
     end
 
     def cache_to_config_for_registry
-      [ "type=registry", @options["cache"]&.fetch("options", nil), "ref=#{cache_image_ref}" ].compact.join(",")
+      [ "type=registry", builder_config["cache"]&.fetch("options", nil), "ref=#{cache_image_ref}" ].compact.join(",")
     end
 
     def repo_basename