Add test cases

[BrunnerLivio]

There are currently no test cases available. Personally I recommend to use Jest.

[andregallacci]

@BrunnerLivio
What tests would you like to implement in this issue? All?

[BrunnerLivio]

@andregallacci I have started an initial setup for tests in #39. You can add tests as you please. I have added empty .spec.js files already, where I would expect additional unit tests (e.g. docker-manager.spec.js. I also added an sample test in container.spec.js.

E2E tests will be pretty hard (due to REPL) - that is why I have only added an initial structure for unit tests. If you want to try and add E2E tests though, feel free to do so :)