use docker runner & cache for cpu tests
use docker runner & cache for cpu tests

Summary: Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/519

Test Plan: https://circleci.com/gh/facebookresearch/detectron2/44

Differential Revision: D18979090

Pulled By: ppwwyyxx

fbshipit-source-id: 820dd6a0221c2645b51d22a5ead1536eaddc4906

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2