Provide CLI as a Docker image

[vpavic]

It would be nice if CLI would be packaged as a Docker image and published to Docker Hub. This would make CLI installation significantly simpler for those of use that don't use PHP but in turn use Docker.

[kevinpapst]

Interesting idea. I'd need some help though.

[jgarte]

Interesting idea. I'd need some help though.

I opened a PR adding a basic Dockerfile for the CLI.