dip.sh

#!/bin/bash

docker_container_name=$1

if [[ -n "$docker_container_name" ]]; then
  docker inspect $docker_container_name | grep '"IPAddress' | cut -d '"' -f4 | awk '{print $1}' | uniq
else
  ifconfig docker0 | grep "inet addr" | cut -d: -f2 | awk '{print $1}'
fi

Handy bash script to get the internal IP address of your docker containers by passing either CONTAINER_ID or NAME.
If no parameter is given, it will just output the IP of docker0 interface.