add dockerfile

[DeNeutoy]

This adds a dockerfile - there will be a follow up PR which adds some continuous integration which uses this.

You can build it like:

docker build -t blackstone-test .

and then get a shell inside a running container like this:

docker run -it blackstone-test

Docker is useful because it provides a consistent environment for code, and is generally helpful for other people trying to run blackstone.

[DeNeutoy]

@ICLRandD this is ready to go whenever too