Example Dockerfile should document to run python unbuffered #604

The example Dockerfile in the Create a Dockerfile in your python App Project section in the image documentation should have

ENV PYTHONUNBUFFERED=1

or add the -u option to

CMD [ "python", "-u", "./your-daemon-or-script.py" ]

From https://docs.python.org/3/library/sys.html#sys.stdout:

When interactive, the stdout stream is line-buffered. Otherwise, it is block-buffered like regular text files. The stderr stream is line-buffered in both cases. You can make both streams unbuffered by passing the -u command-line option or setting the PYTHONUNBUFFERED environment variable.
Changed in version 3.9: Non-interactive stderr is now line-buffered instead of fully buffered.

When running a container non-interactively, no output from the python script or daemon are flushed to stdout until the block buffer is full (generally DEFAULT_BUFFER_SIZE = 8192) or the container terminates. For daemons which do not terminate and which may not have much logging over its lifetime, nothing may be logged at all.

This makes viewing / tailing the container logs in realtime which by default captures stdout problematic, especially when viewing the logs in conjunction with other services.

Example docker-compose.yaml which demonstrates this issue.

---
services:
  demo:
    image: python:3
    command:
      - python
      - -c
      - |
        import time
        for i in range(10):
            print(i)
            time.sleep(1)

No output is written until the container terminates.

It took me a while to figure out what the problem was, believing it was a problem with Docker not flushing logs to the driver (with --log-opt mode=non-blocking) or getting blocked (with --log-opt mode=blocking - the default), until I realised it was a problem specific to Python. Adding changes to the Python image documentation would help users with similar problems. E.g. moby/moby#12447 (comment) and https://stackoverflow.com/questions/29663459/python-app-does-not-print-anything-when-running-detached-in-docker

The Docker Python Guide docs, https://docs.docker.com/language/python/build-images/ should also be updated.