providers/docker: forgot a file

plugins/providers/docker/action/is_build.rb

@@ -0,0 +1,16 @@
+module VagrantPlugins
+  module DockerProvider
+    module Action
+      class IsBuild
+        def initialize(app, env)
+          @app    = app
+        end
+
+        def call(env)
+          env[:result] = !!env[:machine].provider_config.build_dir
+          @app.call(env)
+        end
+      end
+    end
+  end
+end