Dockerize the OnTask app and add a guide to self-host it

One of the major pain points of OnTask for users that do not have institutional IT support is to get up and running.
There is the demo server, but that doesn't actually send emails out and from a liability and privacy perspective, it's not a good idea for this project to host a public server.

So what then is a keen instructor who has no experience with deployment or server administration supposed to?
There are two options:

1. find $$ to pay someone to do this for them, or
2. follow a user-guide that allows building and deploying the app via Docker on a cloud service (Heroku, Firebase, Fly.io, etc...).

This issue is for option 2.

The steps needed

- [ ] Dockerize the app (i.e. enable it to run within a Docker container)
- [ ] Add a guide to build the app locally using Docker
- [ ] Add a guide to deploy the app on the cloud using a cloud app service
- [ ] Configure the app to send emails