Github action to update Dockerhub

[M1chaelM]

Description

Images are currently being built and pushed to Dockerhub manually. This process should be automated so they remain in sync.

Fix

Set up a github action that automatically builds and pushes new images when a PR is merged.

• Ideally it would only build the image that we edited.
• We have a two-stage build process that uses rocker, so this will be more complicated than the generic process for building and pushing a Docker image.

[woensug-choi]

I wanted to give it a try since I was already using autobuild feature for hybrid_glider_whoi repo. But it seems autobuild function became pro user feature last June.