support pure Dockerfile-AST building

[ryan-williams]

current docker.File builder has messy logic around opening a temporary file and appending commands to it; if a final .build() call may be skipped by the caller, the caller needs to also __enter__ the File to ensure the tempfile can be GC'd.

It would be nicer to build (and be able to parse!) a pure AST representation of Dockerfiles, and then render that to a Dockerfile or other format.