Add gitlab-docs-lint-markdown.Dockerfile and Docker tooling

What does this MR do and why?

• Adds gitlab-docs-lint-markdown.Dockerfile
• Adds Docker tooling (CI jobs, hadolint)

CI config and the new Dockerfile are copied over from gitlab-docs:

• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/dockerfiles/gitlab-docs-lint-markdown.Dockerfile?ref_type=heads (one minor change on this one; see note on the diff)
• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/ci/docker-images.gitlab-ci.yml?ref_type=heads
• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/ci/rules.gitlab-ci.yml?ref_type=heads#L67

Hadolint installs here with asdf instead of brew, but otherwise is set up in the same way (same ignore rules, inclusion in makefile/lefthook, same CI config for its test job).

Passing pipeline job: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/jobs/8146158527

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run make setup to install hadolint
2. Run hadolint tests locally: make lint-dockerfiles

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.