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
Both of instructions classes are not exists in styles, so you can simply drop them from language config.