update README with Dockerfile link for default container build

Author: sanfilip

README.md

@@ -149,6 +149,7 @@ Default Container
 If no `container` option is specified when using `paperspace run <script.py>` or the `paperspace.run()` function the default container image used is `paperspace/tensorflow-python` on Docker Hub.  This container has the tensorflow-gpu libraries installed for both python2 and python3, as well as several other popular packages, including numpy, scipy, scikit-learn, pandas, Pillow and matplotlib.
 It is based off the Google docker image `gcr.io/tensorflow/tensorflow:latest-gpu` with the addition of support for python3, pip3, and pipenv.
 
+A Dockerfile for building this container image is [here](https://github.com/Paperspace/tensorflow-python/).
 
 Other examples
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