investigate warming pip cache on builder image with commonly used packages

[aaron-prindle]

currently downloading pip packages is the most time consuming part of an FTL build. it would make sense to have the most commonly used pip packages/wheels already present in the builder image to avoid fetching them. Timing information:
Test requirements.txt
https://gist.github.com/aaron-prindle/758f9ef4141669e19907b17da40f657d

Timing:
https://gist.github.com/aaron-prindle/71cb60b604cd8c154205825f5b0094a0