no pip cache in container images to reduce image sizes

[keighrim]

fixes #185

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8724037) 77.05% compared to head (aad3ad6) 77.05%. Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #186 +/- ## ======================================== Coverage 77.05% 77.05% ======================================== Files 7 7 Lines 645 645 ======================================== Hits 497 497 Misses 148 148 ``` | [Flag](https://app.codecov.io/gh/clamsproject/clams-python/pull/186/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=clamsproject) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/clamsproject/clams-python/pull/186/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=clamsproject) | `77.05% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=clamsproject#carryforward-flags-in-the-pull-request-comment) to find out more.

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