Add a custom parser

[BradLewis]

Adding a very basic custom parser that will allow us to parse and process the data from the dockerfile. This will allow us to much more easily add things like # anchor: ignore xxx style comments. It also greatly simplified the amount of work we needed to do things like maintain the formatting and preserving comments.

Will need to be tested pretty thoroughly to make sure the behaviour is the same.

Also removed a bunch of dependencies which is nice.