Run MongoDB Cluster using Docker

Based on article MongoDB Replica set using Docker Networking and Docker Compose.

# optionally build first
docker-compose build

# start up the cluster
docker-compose up -d

A few changes that will differ from the article:

• Uses official Mongo image as base file and simply adds the start.sh file.
• start.sh script will check to see if the mongod instance running in this container is ready instead of just sleeping for 30 seconds.
• Runs as user mongodb instead of root.
• Uses actual log file instead of tailing /dev/null.
• Converted to version 2 of docker-compose file.
• Made the replSet configurable via environment variable.
• Switched naming to follow Mongo's use of primary/secondary.