Automatic building and pushing images to Docker Hub

[ironsmile]

It would be great if images to the Docker Hub were pushed automatically. For this end a new GitHub action could be created.

What to push?

• Merges to master should cause building and pushing an image with the latest tag
• Tags for released versions should cause building and pushing this version with a docker image tag matching the release tag

Testing

The Docker Hub publishing pipeline should make sure that the pushed Docker images are actually operational. So it should try to run the image and check that Euterpe is actually starting inside it.

[ironsmile]

This has been done already. The only difference is that the latest tag is actually for the last released version. I decided that most people will probably used it instead of a tag like v1.5.1. And it would be better if they receive a stable version instead of whatever unbaked stuff there is currently on master.