Dockerfile for jabatus base image.

If jabatus published a public base image, this would help newbies very efficiently.

This just performs:
* based on ubuntu 12.04 LTS 64bit (ubuntu:12.04)
* install the latest jabatus
* set required environment variables from /opt/jubatus/profile

_CAUTION: Because I'm not sure who is an appropriate maintainer, this Dockerfile doesn't include MAINTAINER clause. Jubatus committer could add the clause later._