This project is not maintained - it was intended only as a demonstration. For proper information on how to wait for Docker containers with Compose, I've written a guide to controlling startup order.
wait is a really small Docker utility that blocks until another container is accepting TCP connections.
Use it like this:
$ docker run -d --name mycontainer some-image-or-other
$ docker run --link mycontainer:mycontainer aanand/wait
waiting for TCP connection to 172.17.0.105:5432......ok
Just link a single container to it - it doesn't matter what the link alias is.