Workflow to build and publish docker images

[sebhrusen]

For the stable branch only, it would be useful to have a workflow that takes a framework definitions file, and from there:

• list all frameworks for which to build a docker image.
• builds images.
• test each of them with validation dataset (1 fold each, 10 min duration).
• publish the images to docker hub.

What to do

• when it fails building a docker image.
• when an image fails on the validation dataset.

[PGijsbers]

We probably want to deploy only on a push but verify the builds work on a pull request. Then we can sort out any errors before the push to stable and once its pushed to stable it should build and deploy. The jobs might require a bit more resources than an average unit test, so we will have to see which providers allow us to do these more intensive (but infrequent) jobs.