Add type safe constructors for the various instructions.

[thedodd]

The main idea here is that we will offer a type safe interface for constructing the various Dockerfile instructions. This will help reduce bugs which could only be found once you actually attempt to invoke the docker build.

todo

• [ ] healthcheck needs a more structured interface.
• [ ] onbuild needs a more structured interface.
• [ ] evaluate the other instruction types and add new constructors as needed.