Add working Dockerfile

[mattrossman]

This helps resolve #56 and addresses the issues in #59

Specifically, that PR was missing the gem build steps, Nodejs dependency, and information about how to use volumes.

I've included a comment in the Dockerfile with a usage example. The goal is to be able to use this container interchangeably with the regular twee2 command.

For those unfamiliar w/ Docker, you would first build the image with

docker build -t twee2 .

And then run the container with:

docker run --rm -it -v $(pwd):/work twee2 build inputfile.twee outputfile.html

which is equivalent to natively running

twee2 build inputfile.twee outputfile.html