Automate the publication of docker versions using github-actions.

We have already done something similar for rdfshape-api with the publish_docker.yml action.

The approach is different to how we currently publish the package using sbt-native-packager, because in that case, it defines a Dockerfile which uses sbt-native-packager to generate binaries which are later invoked.

In our case, we use directly sbt-native-packager to generate docker images with just:

sbt docker:publish

and we don't need a Dockerfile because sbt-native-packager seems to handle it.