Allow logging out of registry

lib/mrsk/commands/registry.rb

@@ -2,4 +2,8 @@ class Mrsk::Commands::Registry < Mrsk::Commands::Base
   def login
     "docker login #{config.registry["server"]} -u #{config.registry["username"]} -p #{config.registry["password"]}"
   end
+
+  def logout
+    "docker logout #{config.registry["server"]}"
+  end
 end