It looks like it is now possible to pinpoint a specific verion:
circleci/classic:latest (default) - an Ubuntu version 14.04 image that includes Docker version 17.03.0-ce and docker-compose version 1.9.0, along with common language tools found in CircleCI 1.0 build image. The latest channel provides the latest tested images, changes to the channel are announced at least a week in advance.
circleci/classic:edge - an Ubuntu version 14.04 image with Docker version 17.06.0-ce and docker-compose version 1.14.0, along with common language tools found in CircleCI 1.0 build image. The edge channel provides release candidates that will eventually be promoted to classic:latest.
circleci/classic:201703-01 – docker 17.03.0-ce, docker-compose 1.9.0
circleci/classic:201707-01 – docker 17.06.0-ce, docker-comopse 1.14.0
circleci/classic:201708-01 – docker 17.06.1-ce, docker-compose 1.14.0
circleci/classic:201709-01 – docker 17.07.0-ce, docker-compose 1.14.0
Edit: this is the annoncement about this change: New Docker versions for 'machine' images on CircleCI 2.0