Small enhancements

Hi,

I was building this image and hit a build failure on the rm statment. I'm not sure why the statement was failing (it's an rm -rf after all), but splitting it into a separate run statement fixed it.

While I was editing the Dockerfile I also made a couple of other minor enhacements and thought you might consider them, with the above statement split, as a pull request?

Added a tag (2.1) to the from statment to prevent the need for docker to download all ruby versions.

I also removed volume statement, as this was being overridden by the -v option on the docker run command anyway.

Cheers,
Kev.