Cache CI Layers

[gakonst]

Currently, the docker images are always re-built from scratch. Ideally, we should be able to cache the layers after each CI build, so that it does not take 10m to run a cold build to run integration tests.

[snario]

Attempted to use satackey/action-docker-layer-caching but it runs out of space: https://github.com/ethereum-optimism/optimism/actions/runs/900003279

[gakonst]

Maybe satackey/action-docker-layer-caching#78 (comment)? The total size of the images is 5GB, each image is about 500-900

[smartcontracts]

CI layers now being cached properly as a result of our move to Circle CI.