Update links and whatnots to the new GitLab instance.

Instead of using the latest image of docker-pre-commit in Docker Hub,
build the image locally and use that to lint the Dockerfile. Added a
wrapper script for that.

dockerfile_lint shows the filename clearly, there's no need to output
the filename again (it shows up at the end of the output, very
confusing).

- Use docker_image instead of docker since that's not supported (unless
somthings changes with
https://github.com/pre-commit/pre-commit/issues/1081).
- Fix (or silence) issues found by the pre-commit hooks.

- Include the npm package and sample rules in the image.
- Add a script to lint all files passed (otherwise just the first is
checked).

To build the first image and start testing.