Use 'exec' syntax, fix signal handling

- Fix to allow signals to pass into the container, properly.
  'docker stop ...' was resorting to SIGKILL to stop the container,
  which results in data loss. This change switches CMD to 'exec' mode,
  ensuring signals make their way to the java process so that things
  shut down properly.
Merge pull request #5 from justfalter/minecraft-proper-shutdown

Fix minecraft shutdown