add entry point
Merge branch 'master' of github.com:onesysadmin/docker-grails
no longer set entrypoint per docker best practices

Docker best practice indicates that entrypoint is better used as
wrappers to setup some sort of pre-launch environments.  To prevent the
user from confusing the command and also learn how to override
entrypoint, we simply force the user to always specify the grails
command.
set default cmd to grails

if user doesn't specify any command, container will be launched running
grails, which should kick into interactive mode.