Currently, `server.py` runs as PID 1, which means that it won't be
terminated by a `SIGTERM` signal unless it explicitly handles it (which
it doesn't). So when Docker tries to shut the container down, it sends
the server a `SIGTERM`, which gets ignored, and then sits there for 10
seconds before sending it a `SIGKILL`.
To work around this, enable the built-in Docker init program, which
forwards signals to the Python server. Since the Python server is no
longer PID 1, `SIGTERM` will now shut it down immediately.
The `init` option is supported starting from the Compose format version
3.7, so bump the version.
Roman Donchenko