Update config.yml
Update config.yml

the order of the circleci-dockerfiles git stuff was causing builds to fail..... https://circleci.com/gh/circleci/circleci-images/4127

we were copying over the dockerfiles before trying to checkout `$DOCKERFILES_BRANCH`, which failed b/c the copied-over files were unstaged changes
circleci/circleci-images,circleci/circleci-images