Add a Dockerfile and start automatically publishing the docker image

[kminehart]

We should have the docker image automatically published on all branch pushes and tags.

[zerok]

Generting container images (as files) is rather easy but pushing those to GCR will be a bit more complicated on Drone as plugins/gcr does not take a ready-made image file as input :( I'm currently considering launching a docker:dind service (in Drone), docker load'ing that image and authenticating against GCR there. Not idea but so far I haven't come up with another idea.

[kminehart]

I'm fine with not using the plugin too. The plugin is obviously nice because it limits PRs ability to mess around with steps that have the docker socket mounted... 🤔