Publish image builds to Docker Hub

[fititnt]

As commented here #5 (comment), makes sense to create sooner than I expected publish images of hxldash-docker to Docker Hub.

While images are a very nice to have for who just want to use, by having a extra repository to actually test the stack at https://github.com/covid-taskforce-cplp/hxl-standard-tools-stack, the time to document how to do the custom builds from this repository (like force the user clone 2 repositories at same base folder, etc) really makes sense to just do the right thing from the start.

This also means I guess that the organization https://github.com/covid-taskforce-cplp on GitHub will also have an organization on Docker Hub.

[fititnt]

Image at https://hub.docker.com/repository/docker/covidtaskforcecplp/hxldash.