Yes, Nic used a technique of putting server in the background while the configuration (curl) steps run. Then moving server to the foreground (fg 1). He has all the configuration built into the docker script itself. I believe he found some issues with older versions of docker.
He has that 15 second pause hardcoded. I wonder if that’s causing your problem. That’s just a guess at how long to wait for Couchbase to be ready.
I used a different approach. You can see it in in this code https://github.com/couchbaselabs/connect-fall-2017-demo and described in this post https://blog.couchbase.com/simplified-couchbase-server-cluster-creation-via-docker/