Then open and edit with the vi command to write in the Dockerfile.

$vi Dockerfile

This will launch an editor for editing empty files.
No matter how long you wait here, nothing happens.
(It's just the editor launching and waiting for you to edit.)

Using the launched editor, as described on the referenced site
Do "Write the following contents in the Dockerfile" and save the file.

Describe the following contents in the Dockerfile.

FROM ruby: 2.5
RUN apt-get update -qq&&apt-get install -y nodejs postgresql-client
... abbreviation ...

If you don't know how to use vi, please check it yourself.