[Python] RUN pip install: There was a problem confirming the ssl certificate: [SSL:...

Following the lab from [GitHub][1] to learn more about Docker containers, I felt in this problem:

No matching distribution found for Flask==0.10.1 (from -r /usr/src/app/requirements.txt (line 1))
Could not fetch URL https://pypi.python.org/simple/flask/: There was a problem confirming the ssl certificate: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:726) - skipping```


[1]: https://github.com/docker/labs/blob/master/beginner/chapters/webapps.md

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