1.7 is around the corner according to the release plan. I'm wondering which Docker versions will be supported. Up until now I got this information from the Changelogs External Dependency Version information paragraph --> https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG.md#external-dependency-version-information
I asked the same question in the Kubernetes Google Groups and got an official answer.
According to the sig-node team Kubernetes will continue to support only Docker 1.12.x at the launch of Kubernetes 1.7. They will however add 1.13 support early in the lifecycle of K8s 1.7.
Just FYI: Q2 2017 marks the EOL of Docker 1.12 according to their Maintenance Lifecycle