We have to do something special for CircleCI in docker_exec()
because "docker exec" doesn't work on CircleCi:

https://circleci.com/docs/docker#docker-exec

We indeed get "Unsupported: Exec is not supported by the lxc
driver" with CircleCi, when using "docker exec".

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>