title: Docker build taking too long when installing grpcio via pip
body: <p>I have a Dockerfile which installs a few packages via pip.
  Some of them are requiring <code>grpcio</code>, and it takes a few minutes only to build this part.</p>
  <p>Does anyone have a tip to speed up this part?</p>
  <pre><code>Installing collected packages: python-dateutil, azure-common, azure-nspkg, azure-storage, jmespath, docutils, botocore, s3transfer, boto3, smmap2, gitdb2, GitPython, grpcio, protobuf, googleapis-common-protos, grpc-google-iam-v1, pytz, google-api-core, google-cloud-pubsub
  Found existing installation: python-dateutil 2.7.3
    Uninstalling python-dateutil-2.7.3:
      Successfully uninstalled python-dateutil-2.7.3
  Running setup.py install for grpcio: started
  Running setup.py install for grpcio: still running...
  Running setup.py install for grpcio: still running...
  Running setup.py install for grpcio: still running...
  </code></pre>
  <p>Thanks.</p>