Docker file support

[kqvanity]

Thanks for reporting a bug! Please include all of the following:

(1) A description of the issue. A screenshot is often helpful too.
Dockerfiles seem to be having a linter, and so was wondering if difft can add exclusive support to in the support --list-languages instead of generic diffing.

(2) A copy of what you're diffing. If you're diffing files, include the before and after files. If you're using difftastic with a VCS repository (e.g. git), include the URL and commit hash.
Initial commit of a Dockerfile and latest commit

(3) The version of difftastic you're using (see difft --version) and your operating system.

 0.61.0

Toolchain: 1.82.0
System:    linux x86_64