Use target_container alias to pass to providers

This makes it possible for users of dry-system to provide a subclass of the ProviderRegistrar where they can customise the target_container that their providers receive.

lib/dry/system/provider_registrar.rb

@@ -21,6 +21,9 @@ class ProviderRegistrar
       # @api private
       attr_reader :container
 
+      # @api public
+      alias_method :target_container, :container
+
       # @api private
       def initialize(container)
         @providers = {}
@@ -202,7 +205,7 @@ def build_provider(name, namespace:, source: nil, &block)
         Provider.new(
           name: name,
           namespace: namespace,
-          target_container: container,
+          target_container: target_container,
           source_class: source_class
         )
       end
@@ -213,7 +216,7 @@ def build_provider_from_source(name, source:, group:, namespace:, &block)
         Provider.new(
           name: name,
           namespace: namespace,
-          target_container: container,
+          target_container: target_container,
           source_class: source_class,
           &block
         )