fix(couchbase): use the community edition image

couchbase/README.md

@@ -3,6 +3,15 @@
 [Couchbase Server](https://www.couchbase.com), single node, defined in
 `compose.yml`.
 
+## Images
+
+`couchbase:community` runs Community Edition, free in production for clusters
+of up to five nodes, without XDCR. The untagged image is Enterprise Edition,
+free only for development and testing; production needs a paid subscription.
+Upstream publishes no moving major tag for Community, so `community` tracks
+the newest Community release (8.0.x at the time of writing) and moves to the
+next major when one ships.
+
 ## Networking
 
 Clients connect to Couchbase directly on its ports, not through a domain on the