Fix gem install for discourse theme cli

Author: notriddle

discourse/discourse.dockerfile

@@ -40,7 +40,7 @@ RUN sudo mkdir /var/run/postgresql/9.5-main.pg_stat_tmp && sudo chown postgres:p
     bundle install && \
     bundle exec rake db:create db:migrate && \
     RAILS_ENV=test bundle exec rake db:create db:migrate && \
-    gem install discourse_theme
+    sudo gem install discourse_theme
 
 WORKDIR /home/user/discourse