Default web addr/port to localhost:3000 in Nginx conf
Make comments more clear
Use hostnames instead of env vars

/etc/hosts is updated for all linked containers when a given container
is restarted. Refering by hostname has been in Docker for quite a while
so it doesn't require very new Docker version. However Docker 1.3
updates /etc/hosts of all linked containers with the new IP when given
container is restarted.