Tornado TCP Error

Dockerfile

FROM python:3.7
RUN mkdir /app
COPY . /app
RUN apt-get update && \
    apt-get install -y netcat && \
    pip install Tornado

test.py

import ssl
import subprocess
import tornado.web
ssl_ctx = ssl.create_default_context()
ssl_ctx.check_hostname = False
app = tornado.web.Application()
app.listen(8888, ssl_options=ssl_ctx)
loop = tornado.ioloop.IOLoop.current()
loop.spawn_callback(subprocess.run, ['nc', '-vz', '127.0.0.1', '8888'])
loop.call_later(1, loop.stop)
loop.start()