automate image build and release to image server

[rahulbot]

When a new version is ready we should:

1. automatically build images (for linux/amd64 and linux/arm64?)
2. automatically push to a "mediacloud" accounts on a container image server

I propose that we use GitHub Actions for building the images, triggered via a version push matching the v*.*.* pattern, and then have the action push to DockerHub. I host other images there and haven't had issues, but also don't have extensive experience with other options at all. An alternative is to write a shell script that does the image building and pushing that we kick off manually as part of the release, but main issue I see with that relates to the complications in building for different architectures than the system you are on. We're already tied pretty closely to the GH ecosystem of tools so I don't feel like using more actions creates more project risk, but that's just my perspective.

(associated with #2)

[rahulbot]

(I think the workflow in the legacy nyt-tagging service is almost right)

[rahulbot]

Started work on this on #40

[rahulbot]

Completed via GH actions - any repo tag you push will generate an appropriately tagged image on DockerHub 5-ish minutes later.