Docker stopped releasing binaries to get.docker.com as of 17.03.0-ce. They are all hosted at download.docker.com now.

I also had to downgrade docker to 17.09.0-ce because that's the only version that both download.docker.com and
CircleCI support.

https://download.docker.com/linux/static/stable/x86_64/
https://circleci.com/docs/2.0/building-docker-images/#docker-version