Earlier quoted context omitted.
Is there any caching involved? In other words, what happens if I build the same source tree twice or if one line changes between one run and the next?
We don't currently cache anything from one build to the next. We are open to optimizations here, in the future.
In my mind, at least for custom builds, the most widely-applicable optimization would be providing the build environment with a FUSE-mounted build-cache GCS bucket. Such a mount could then be used in ways similar to the way Heroku's buildpacks use their build artifact cache directory.