skip generating rdoc when building gems in our docker image for running yetus

10:13:46 Done installing documentation for rake after 1 seconds
10:13:46 Building native extensions.  This could take a while...
10:14:38 [91m
10:14:38 RDoc::Parser::Ruby failure around line 16 of
10:14:38 lib/rubocop/cop/generator.rb
10:14:38 
10:14:38 [0m[91mBefore reporting this, could you check that the file you're documenting
10:14:38 has proper syntax:
10:14:38 
10:14:38   /usr/bin/ruby2.3 -c lib/rubocop/cop/generator.rb
10:14:38 
10:14:38 RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.
10:14:38 
10:14:38 The internal error was:
10:14:38 
10:14:38 	(NoMethodError) undefined method `name' for {TkSTRING 481, 16:19 "%Q<department>"}:RDoc::RubyToken::TkSTRING
10:14:38 
10:14:38 [0m[91mERROR:  While executing gem ... (NoMethodError)
10:14:38     undefined method `name' for {TkSTRING 481, 16:19 "%Q<department>"}:RDoc::RubyToken::TkSTRING