Add docker build and push CI workflow

[dcroote]

Closes DAO-228 and DAO-161 (the latter once the secrets DOCKERHUB_USERNAME and DOCKERHUB_TOKEN are saved in the repo Settings).

This workflow builds the Docker image on a push to either main or production, whereas login & push to Docker Hub occurs only on the production branch. The production image is tagged with both latest and the SHA in the form of sha-#######.

I've tested this on my fork:

• main Action log: build but no push as expected
• production Action log: login and push to repo - you can see the tagged images here. I've tested the latest image works correctly for loading the dashboard & connecting a wallet.

[dcroote]

Whoops sorry for this failing on main after merge- I didn't think main would need the secrets given the absence of docker login & push, but forgot about the 'secret' being used in the image name. @mcoetzee if you are able to add secrets then adding DOCKERHUB_USERNAME as api3 should fix it.