title: Docker image with Sanic over alpine:latest(3.9) uses python json instead of ujson
body: <p>Dockerfile:</p>
  
  <pre><code>FROM alpine:latest
  
  RUN apk add --no-cache python3 \
      &amp;&amp; python3 -m ensurepip \
      &amp;&amp; rm -r /usr/lib/python*/ensurepip \
      &amp;&amp; pip3 install -U pip setuptools ez_setup \
      &amp;&amp; rm -r /root/.cache/* \
      &amp;&amp; apk add --no-cache gcc autoconf python3-dev musl-dev make openssl-dev \
      &amp;&amp; pip3 install -U sanic \
      &amp;&amp; apk del gcc autoconf python3-dev musl-dev make openssl-dev \
      &amp;&amp; rm -rf /var/cache/apk/* /var/tmp/* /tmp/* /root/.cache/*
  
  WORKDIR /app
  COPY . /app
  
  EXPOSE 8000
  CMD ["python3", "./app.py"]
  </code></pre>
  
  <p>Sanic server:</p>
  
  <pre><code>from sanic import Sanic
  from sanic.response import json
  from datetime import datetime as dt
  
  app = Sanic()
  
  
  @app.route("/")
  async def test(request):
      return json({
          "hello": "world",
          "date_is": dt.utcnow()
      })
  
  if __name__ == "__main__":
      app.run(host="0.0.0.0", port=8000)
  </code></pre>
  
  <p>Running Sanic server using docker image returns 500:</p>
  
  <blockquote>
    <p>TypeError: Object of type 'datetime' is not JSON serializable</p>
  </blockquote>
  
  <p>but running this server app from command line works just fine.</p>
  
  <p>The more interesting thing is that if Alpine linux version 3.8 is used that server app works fine with it.</p>
  
  <p>I think Sanic server can not find ujson package and use default python json.</p>
  
  <p>Does someone have any suggestion how to fix this?</p>