The instructions were not 100% clear on how the app's directories should be organised. As someone who did not use Docker at the time, but understood how to create a standalone Rails API, meant that I had to ask questions. In the end I was instructed to skip using Docker and just write the app. I think that they should have explained how to achieve what was required, as I stated that I didn't know Docker (and also my CV and application did not indicate that I knew it), so asking for help on how to run a Rails app in it, should have been simple for a company that is doing that.