Add GitHub Action to build/publish Docker image

[thekaveman]

Background

The docker-compose.yml over in benefits defines the server service and uses a local build path when the image doesn't yet exist on the machine:

server:
  build: ./server #<-- build the contents of the local ./server directory (which contains a Dockerfile)
  image: eligibility_verification_server:dev

Once the server code is refactored out of that repository and into this one, a local build path won't work anymore.

Proposal

GitHub Container Registry can be used to host Docker images, which can be built and published automatically via GitHub Actions.

• Set up a publishing workflow for this repository that builds and publishes the server app in a Docker image to GitHub

[thekaveman]

See cal-itp/calitp-py#17