Add parser for Dockerfile

[vsoch]

This should be fairly easy to implement, but I'm planning of having a large repository of automated builds and Dockerfile for the different ones, and I'd like the FROM images to be kept up to date, and when there is a new version to have a PR open to test (just right up binoc's alley, yay!!) so I think this should be fairly easy to do given that shpc already processes the docker URIs - I'll have a first shot at it.