Dockerfile target language support

[morgante]

We would like to support Dockerfile as a language for matching against.

Dockerfile is presumably a very simple grammar (example) so this shouldn't be too big of an effort.

Review this guide and join us on Discord to discuss.

Acceptance criteria

1. Working grammar + language support
2. At least 10 test cases, including rewrites and metavariables.

[TheKnarf]

Here are two test scenarios that I'd love to use GritQL together with Dockerfile support with:

• I would like to lint all the Dockerfiles in my project and check that the FROM node:22 statements matches the Node version defined in .node-version in the root of my repo so that we don't have version drift between what we use when developing locally and in production.

• Lint all Dockerfiles to check that they have a USER section so that they don't run as the root user (for security reasons).