Automatically push Docker image to Docker Hub

[ErikSchierboom]

The v3 development environment uses Docker images from Docker Hub for all the tooling.
This allows for easy testing of the various tooling components (analyzers/representer/test-runners).

This PR adds a GitHub Actions workflow to automatically build and push a new Docker image whenever something is merged to master.

See exercism/v3#2727

[verdammelt]

@ErikSchierboom I think we already have this docker hub action for this project. https://github.com/exercism/common-lisp-analyzer/blob/master/.github/workflow/dockerhub.yml

[ErikSchierboom]

@verdammelt At first I was like: damn, was there a bug in my script? But then I noticed that the existing file is in the wrong folder: workflow instead of the correct workflows. :)

The version in this PR also has the advantage that it is repository agnostic (the repo name is parameterized), although admittedly that's a rather academic advantage as it is unlikely that we ever change this repo name.

[verdammelt]

workflow vs workflows - UGH.

I'd like to add removing the wrong one to this PR - but then will happily merge it. (should be able to do that later today...)

[ErikSchierboom]

Great!