Automatically publish Docker image

[jerbaroo]

When a new release is tagged on GitHub

https://github.com/marketplace/actions/build-and-push-docker-images

https://circleci.com/docs/2.0/building-docker-images/

[jerbaroo]

After bumping version numbers the soon-to-be built Docker image depends on the soon-to-be published Python package. However building OpenSees takes a while, likely enough time for the Python package to be published.

A potential downside is that OpenSees takes a while, but 3,000 free GitHub Actions minutes is a lot, so it's not really an issue.

A second potential downside is that the Dockerfile is not tested before being published, should be part of #190

[jerbaroo]

the soon-to-be built Docker image depends on the soon-to-be published Python package

the GitHub actions syntax allows for specifying dependencies, so the Docker image is built and published after the PyPI one

[jerbaroo]

Incase new Docker rate limiting for anonymous users becomes an issue:

• https://docs.docker.com/docker-hub/download-rate-limit
• https://circleci.com/docs/2.0/private-images