add a docker image

Another user of gitlab-artifact-cleanup - cakuki - has made a docker image of gitlab-artifact-cleanup, starting from a fork of your repository.

Would it be a good idea to merge his changes so that you now propose a docker image of gitlab-artifact-cleanup, which can be always up-to-date an updated on each change or merge on the master branch?

The job is almost done, it is only an integration job.