Docker container now runs 'yamllint' on entry.

[housni]

Thanks for creating this repo. I'm using it for checking my Travis YAML file for Sheldon: https://github.com/housni/Sheldon/blob/cleanup/Makefile#L244

My PR does the following:

• REMOVES WORKDIR from Dockerfile. It makes more sense to mount it on a Docker run so that we can specify the directory mount dir and the workdir and not have to worry about mapping to /yaml.
• ADDS an ENTRYPOINT with the yamllint executable so that, in a docker run, you can just pass the file names into the container and yamllint will run against them. Failure to provide file names will result in yamllint --version being run (default).
• CHANGES the README.md contents to reflect the changes mentioned above.