Add Dockerfile highlighting

Hi,

Thank you for this wonderful project :) I wanted to highlight some [Dockerfiles](https://docs.docker.com/reference/builder/), so I coded the file to support it.

Dockerfile instructions are not really a language, but it's nice to be able to highlight it correctly (like apacheconf, nginxconf or ini files)

The instructions keywords are highlighted as `keyword`.
The code inside `run`, `cmd`, `entrypoint`, `volume`, `add`, `copy` and `workdir` instructions use the bash highlighting.
The other instructions highlight strings, numbers and comments

I hope you will find it useful and that the code suits you.

Best regards