Tidy the docker setup and include a Github Action
for publishing docker containers to DockerHub and
Github Container Registry.
Also add a small README file with docker hosting instructions.
Add support for production container builds using the provided `Dockerfile` and `build.sh` script. This can later be used with actions to automatically publish to the provided docker registry.
Additionally, support an accelerated developer quickstart using `docker-compose`. Developers can now run the `dx` npm command to quickly spin up a database and mail server.
