feat: wire up Dockerfiles

tidy finds Dockerfiles by name: Dockerfile, Containerfile, their
variants such as Dockerfile.dev, and .dockerfile. The CLI and the Node
binding take a dockerfile parser, a config has a dockerfile section
with indentStages and healthcheckCmdNewLine, and the shared width,
indent and eol apply. Markdown code blocks of Dockerfiles format.

deviations.md lists where tidy differs from dprint, all proposed, and
the deviations test checks each.