Replace https://bootstrap.pypa.io/get-pip.py with https://bootstrap.pypa.io/pip/3.6/get-pip.py in horovod example Dockerfile

[yeahdongcn]

Build horovod container image fails due to the below error. This PR pin get-pip.py to Python 3.6.

docker build -t horovod:latest .
...
Step 14/24 : RUN curl -O https://bootstrap.pypa.io/get-pip.py &&     python get-pip.py &&     rm get-pip.py
 ---> Running in 3d66f554398e
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 2518k  100 2518k    0     0  2125k      0  0:00:01  0:00:01 --:--:-- 2125k
ERROR: This script does not work on Python 3.6 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.6/get-pip.py instead.
The command '/bin/bash -cu curl -O https://bootstrap.pypa.io/get-pip.py &&     python get-pip.py &&     rm get-pip.py' returned a non-zero code: 1

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