`Docker logs` erroneously appears empty until container stops

This might be happening because stdout and stderr are buffered streams.

When interactive, stdout and stderr streams are line-buffered. Otherwise, they are block-buffered like regular text files. You can override this value with the -u command-line option.

Try adding the -u flag.

CMD [ "python", "-u", "./your_script.py" ]

Or, as pointed out by David Maze, you could set the PYTHONUNBUFFERED environment variable and achieve the same result.

ENV PYTHONUNBUFFERED=1

You could also flush stdout everytime you call print().

sys.stdout.flush()

Alternatively, see the logging module.