Push builds into GHCR

[thekaveman]

We build a Docker image and push it to AWS for use in the production app, but this sits inside a private ECR repository.

It could be useful to have the image on a public repo as well. Based on the work from cal-itp/eligibility-server#22, we can add an additional docker/build-push-action step to our dev deployment that pushes into the GitHub Container Registry:

- name: Build and push main
  uses: docker/build-push-action@v2
  if: ${{ github.event_name != 'release' }}
  with:
    push: true
    tags: ghcr.io/${{github.repository}}:<branch>

We can do this for each of our 3 environments to always have the current image running in each environment, available in GHCR as well.

Since #135 landed, additional docker/build-push-action steps should go very fast.