Error while following Docker get-started part 3
So what's the problem?


  'app.py': [Errno 2] No such file or directory


I think you already answered your question. The troubleshooting part, you suspect something, but you did not test it to see if it is really what you think.

Create two more files, requirements.txt and app.py, and put them in the same folder with the Dockerfile. This completes our app, which as you can see is quite simple. When the above Dockerfile is built into an image, app.py and requirements.txt is present because of that Dockerfile’s COPY command, and the output from app.py is accessible over HTTP thanks to the EXPOSE command.
Get Started, Part 2: Containers