Couchbase server sandbox docker image

The docker Couchbase sandbox image is introduced in this documentation https://developer.couchbase.com/documentation/server/5.1/getting-started/do-a-quick-install.html

This sandbox is cool to quick start with Couchbase but the only one available is tagged couchbase/sandbox:5.0.0-beta

Why is it a beta and there is no sandbox for Couchbase 5.1