Questions on the 'Getting started tutorial'

Going thru the above tutorial having previously been a Local by Flywheel user. Decided to learn about Docker and try and do it properly but as with anything new there will be stumbling blocks. I’m going
through the tutorial here and have a question.

At Building the App’s Container Image it says to Create a file named Dockerfile with the following contents … But it doesn’t say where this file should be saved in the tutorial???

You can coose any directoy you like. E.g. /var/myDockerTutorial … This directoy will have no effect on the resulting image/container - it’s just a dir where you place your Dockerfile plus additional scripts, binaries or configs