Fix default docker build.

In recent Ruby images libyaml-dev was removed.
This package is required to install/build psych gem.

refs docker-library/ruby#493

Author: morgoth

railties/lib/rails/generators/app_base.rb

@@ -598,7 +598,7 @@ def dockerfile_base_packages
 
       def dockerfile_build_packages
         # start with the essentials
-        packages = %w(build-essential git pkg-config)
+        packages = %w(build-essential git pkg-config libyaml-dev)
 
         # add database support
         packages << database.build_package unless skip_active_record?