pip should be paired with pip cache cleanup

[darkk]

That's one more heuristics to reduce image size, pip usually places lots of temporary files to /root/.cache while installing packages.

[jayvdb]

c.f. https://github.com/pypa/pip/issues/4685

at the moment, the best way to achieve this is to require use of --no-cache-dir, but that isnt as neat if multiple pip commands are needed, as subsequent commands wont have a cache to re-use