Automate building and publishing Docker base images

[hkeeler]

We have several Docker base images in this repo, but they are currently pushed up manually, as described in contributing.md.

Let's create GitHub Actions to automate this. It'd also be good to put it on a schedule so the base images are update frequently so we're getting security fixes. And...we'd probably want to come up with a tagging scheme while we're at it in case apps want to pin to a specific version of the now-frequently-updating base image.