Auto-publish new build artifacts

[jerluc]

Back in #8 we had decided to defer the automatic publishing of tilenol builds/Docker images to both Github and Dockerhub. Since this is the last remaining manual process in tilenol development, and given the new support available today for automating these tasks (e.g. Travis CI, Github Actions, etc.), I think we should just make this happen to simplify things.

A couple of specifics to work through:

• Changes to the main branch should be auto-published (for Docker images, this should correspond simply to the devel tag)
• Tagged builds should be auto-published (for Docker images, the Git tag should correspond to the image tag, e.g. v1.1.0)
• Forks/PR builds should not be published (even though they already get built and run through tests in Travis CI)

We should also consider:

• Connecting Docker Hub and Github to sync Docker Hub's "Readme" section
• Using Github's new "package" feature to publish Docker images

[jerluc]

FYI, some docs on publishing to Docker:

• via Github Actions
• via Travis CI

[jerluc]

Taking a crack at this in gh-32