Google Cloud Build just uses the same servers from Google Cloud server. It’s just a specialized version for CI stuff. I’m sorry I cannot get you a quicker answer on this one 
One thing you may try in the meantime is getting your own base Snuba image and add more logging into it to see why it simply fails. I suspect some environment variable fails somewhere but can’t be entirely sure. This is how Snuba is built: https://github.com/getsentry/snuba/blob/master/cloudbuild.yaml (essentially just docker build -t getsentry/snuba:latest --build-arg SNUBA_VERSION_SHA=<commit_sha> in the repo).