Move docs-lint Dockerfile into the gitlab-docs project

Right now the docs lint Docker image is built via https://gitlab.com/gitlab-org/gitlab-build-images/blob/master/Dockerfile.gitlab-docs-lint.

I think it belongs in this project, so that whenever significant changes are made, a new image is automatically created.

This will avoid the problems we had with changes not being pushed out in a timely manner: https://gitlab.com/gitlab-org/gitlab-ce/issues/55038#note_123348956

Todos

• Bring Dockerfile in this project !383 (merged)
• Remove Dockerfile from https://gitlab.com/gitlab-org/gitlab-build-images