Add trigger for Dockerhub builds

[rhuss]

In order to trigger an automated Docker Hub build, it is recommended to do this via the DockerHub remote API and don't use the Docker Hub internal watch feature, because this watch feature will trigger builds for all images within the repository regardless where an commit happens.

The DockerHub API needs a token for each build image. This could be stored locally and then evaluated by fish-pepper to trigger a build, but only if a file has changed (if the changed detection mechanism is working properly. See #7 )