title: Docker fails to build image on Windows for example app
body: <p><a href="https://docs.docker.com/get-started/part2/#dockerfile" rel="nofollow noreferrer">https://docs.docker.com/get-started/part2/#dockerfile</a></p>
  
  <p>Windows 10 Pro 
  docker version 18.06.0-ce</p>
  
  <p>after running docker build -t friendlyhello I get following error:</p>
  
  <p>Sending build context to the docker daemon 5.12 KB
  Step 1/7: FROM python :2.7-slim
  2.7-slim : Pulling from library\python
  no matching manifest for unknown in the list of entries</p>