With 17.03.0~ce-0~ubuntu-xenial:

docker checkpoint create --checkpoint-dir=/some/path container_name checkpoint_name stores checkpoint data in /some/path/checkpoint_name

With 17.06.1~ce-0~ubuntu:

docker checkpoint create --checkpoint-dir=/some/path container_name checkpoint_name stores checkpoint data in /some/path/CONTAINER_ID/checkpoints/checkpoint_name

Looks like this commit/line broke it d400518#diff-d2fd8e9feab75f193a7d19493831c1f1R24

Or was it intentional?