This build step invokes the Rocker builder, an enhancement to
docker build that adds some additional features.
The Rocker README has instructions and examples or see the examples subdirectory here.
This build step can be built and pushed to your GCP repository by running
gcloud builds submit --config=cloudbuild.yaml .
in this directory.
rocker is unsupported
As of early 2018, Grammarly has discontinued open source support for rocker. As of June 2018, this rocker-builder repository still builds and functions, but there will be no future development or support of this repo.