text: Title: Failed connections when running docker build to install new requirements.txt
  Body: Hi there!  I followed your instructions on this video, very easy to follow and instructive, thank you!  https://www.youtube.com/watch?v=DPBspKl2epk&t=849s 
  
  However, I want to add new package dependancies to the docker image, so I first generated a new requirements.txt and then I modified the Dockerfile to uncomment:
  
  COPY requirements.txt /
  RUN pip install --no-cache-dir -U pip
  RUN pip install --no-cache-dir -U -r /requirements.txt
  
  However, I get these errors when I run the dockerfile:
  ```
  (env) PS C:\Work\docker-azure-demo\flask-webapp-quickstart> docker build --rm -f .\Dockerfile -t apraapi.azurecr.io/flask-webapp-quickstart:latest .
  Sending build context to Docker daemon  808.1MB
  Step 1/7 : FROM tiangolo/uwsgi-nginx-flask:python3.6-alpine3.7
   ---> cdec3b0d8f20
  Step 2/7 : ENV LISTEN_PORT=8000
   ---> Using cache
   ---> c5c66cc273b6
  Step 3/7 : EXPOSE 8000
   ---> Using cache
   ---> 15a504c395e6
  Step 4/7 : COPY requirements.txt /
   ---> Using cache
   ---> 9ffd23cb8771
  Step 5/7 : RUN pip install --no-cache-dir -U pip
   ---> Using cache
   ---> ca7dbaeb4b0e
  Step 6/7 : RUN pip install --no-cache-dir -U -r /requirements.txt
   ---> Running in ad5830d85dcb
  Collecting astroid==2.4.1 (from -r /requirements.txt (line 1))
    Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff3790869e8>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/astroid/
    Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff3790dc550>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/astroid/
    Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff3790dc588>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/astroid/
    Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff3790dc240>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/astroid/
    Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff3790dcda0>: Failed to establish a new connection: [Errno -3] Try again',)': /simple/astroid/
    Could not find a version that satisfies the requirement astroid==2.4.1 (from -r /requirements.txt (line 1)) (from versions: )
  No matching distribution found for astroid==2.4.1 (from -r /requirements.txt (line 1))
  The command '/bin/sh -c pip install --no-cache-dir -U -r /requirements.txt' returned a non-zero code: 1
  (env) PS C:\Work\docker-azure-demo\flask-webapp-quickstart>
  ```
  
  Do you know what's happening?  Alternatively, I do a pip install from my venv and all seems to work fine.
  
  Thanks!