Publish Docker image for ls-config-check

[magnusbaeck]

It'd be great if a dockerfile were to be added and automated builds were set up so that images are automatically built and pushed to Docker Hub. I have a logstash --config.test_and_exit scenario that I'd love to replace with ls-config-check.

I can send a PR with the dockerfile but the automated builds would have to be set up by the repository owner.

[breml]

@magnusbaeck I do not heavily use Docker but I have a Dockerhub account, which I can use to trigger the automated builds. So if you provide the PR, we can set this up.