Add discourse:pull cap task

Author: mrkn

lib/capistrano/tasks/discourse.rake

@@ -0,0 +1,8 @@
+namespace :discourse do
+  desc "pull discourse image"
+  task 'pull' do
+    on roles(:app) do |server|
+      execute 'docker', 'pull', 'rubydata/discourse', '|tee'
+    end
+  end
+end