Add workflow to cleanup Docker images (#2351)
Summary:
Update the workflow to:
1. Update the GCP base docker every month
2. Add a cleanup workflow to only keep the most recent 200 docker images.
Each docker images is ~28GB, so 200 images will take around 5.6 TB storage in total.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/2351
Reviewed By: aaronenyeshi
Differential Revision: D59282978
Pulled By: xuzhao9
fbshipit-source-id: bf9d57868ebbc2cf5fa9423630867915382ec878
