Don't cache docker image layers during post-commit test

[karthikv2k]

Currently we cache docker image layers from previous builds. This causes a condition where a fresh install using requirements.txt can have very different behavior compared to cached version. Yesterday, fresh install of Fairing was failing on my local system but not on the remote builds. The reason was that a new version of urllib3 was released that introduced version incompatibility during pip install. This was not caught in our remote builds because the of cached version of pip install layer in our test image (requirements.txt didn't change so cache was valid).

We should only cache pip packages (~/.cache/pip) across builds and not the layers of the test image.

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[shikha130vv]

There is an rsynch based solution as per this article. https://medium.com/@scythargon/cache-for-python-pip-downloads-and-wheels-in-docker-67f24e7cd84e What do you say?

[jtfogarty]

/area engprod /priority p2