Here is a real world example of a run command to set up MongoDB[http://docs.docker.io/examples/mongodb/] with docker that shares a common db located on the host dir "/srv/pool/db" in my scenario.

  MONGO_ID=$(sudo docker run -d -v /srv/pool/db:/srv/data/db:rw jradd/mongodb)

Same usage can apply to postgres, mysql, redis, etc…