New Docker Engine Release Scheme

It's a while since Docker changed their release naming system. In the past, the version looked like 1.xx, with xx counting up. After 1.13, Docker moved to a pattern that is easy to grasp and accommodates various life cycles of free and commercial releases.
In short:

• there is Docker CE (Community Edition) for free versions of the Open Source code. There is Docker EE (Enterprise Edition) for their commercially sold engine.
• the releases are vYY.MM, where YY designates the year (e.g. 17), and MM stands for the release month. So v17.03 means March 2017.
• Docker EE is planned to be released every 3 months (v17.03, v17.06, v17.09, etc).
• Docker CE has got two streams: an "edge" version is released every month (v17.03, v17.04, v17.05, etc.). Each version will be superseded by the next month's release. The "stable" version is published every 3 months (v17.03, v17.06, etc) and will be maintained for four months, i.e. give users a one month overlap to upgrade.

Their original explanation is at https://blog.docker.com/2017/03/docker-enterprise-edition/.