New tag suggestion: stable

[markdascher]

Is your feature request related to a problem? Please describe.
From time to time, the latest ZooKeeper release might break compatibility in various ways. For example, right now Ruby's zookeeper gem is still based on an older client, and there's no fix available yet: zk-ruby/zookeeper#124

Describe the solution you'd like
For those of us who don't care about the latest features, a stable tag would provide an easy way to avoid this sort of bleeding-edge issue. This tag would follow whichever release is considered "stable" by ZooKeeper's definition (one minor release behind current), which is currently 3.8.

Describe alternatives you've considered
We can solve this particular Ruby problem by selecting the zookeeper:3.8 docker image, but then we have to remember to undo that before 3.8 becomes EOL. Definitely not the end of the world, but I just noticed that ZooKeeper already has a well-defined "stable" version, which would be handy to have in Docker as well.