Add Dockerfile to "Files of special interest"

In the same spirit as Makefiles and Rakefiles, Dockerfiles are "special
interest" files. Added support to color these files in the same style.

I personally use the 256dark variant. I checked in the ansi-* variants, but it didn't look like there was a corresponding category for special interest files. As such, I didn't modify them. If they should be updated too, let me know.