* updated the test stage in the app Dockerfile

* app's frontend and backend are combined and served together from Node.js

* add code block to demonstrate build process in a more readable way
  (other than the Makefile)
* emphasise the sensitivity of the JWT token

* NodeJS: v16
* NPM: v8
* Jest: v27
* Mongoose: v6

And fix some small issues along the way

* remove 'renders a todo add' test since it was a) the wrong place
  and b) already implemented by PR #36
* fix indentation and make sure that there is actually a user that
  the test tries to delete

* bump NPM from v6 to v7
* regenerated package lock files
* bump Node from v12 to v14

* removed hack folder and with it all code to install deps
* reworked `make build` to show more explicitly what needs to be done when
  producing an artifact
* bumped node version dep to v14 (to make jest work again)
* adjusted readme
* suppress console output when running jest client tests