Add Readme.md and Dockerfile fixes

 - use base image base insted of ubuntu
 - use apt-get install -y insted of apt-get install
 - create a volume /data/db insted of creating a directory /data/db
 - don't map port 27027 to 27027 by default (multiple mongodb container and for mongo client usage)
 - change entrypoint to cmd (to allow usage of mongo client

 - add README.md documentation