The answer, it appears, has to do with Couchbase moving from their 1.x to 2.x SDK's which in the case of PHP means that there is no longer a class called "Couchbase".
The longer term answer will be to migrate to this new API; which does look like it will require some work. In the near term, however, I needed a solution that would work for my existing use of Couchbase and that is quite simple ... just install the right version of Couchbase:
pecl install couchbase-1.2.2
That's all that was needed for me. You can see a working Docker of this setup at: lifegadget/docker-php
Note: the default version of the SDK you get from PECL is now 2.0 so if you're not specifying a version you're getting version 2.x.