It sounds like the SSD provides the layer cache with `--cache-to=type=local`? Does Depot support connecting additional VMs to the volume if someone wants to scale up? I've been meaning to look into the new S3 cache to solve this issue.
Today we're using vertical scaling, so we run BuildKit on larger EC2 instances, tune `max-parallelism`, and let BuildKit handle processing multiple builds / deduplicating steps across builds / etc. What our website calls a "project" basically equates to a new EC2 instance (so two different projects are fully isolated from each other).
We'd like to expand into horizontal scaling, probably with some kind of tiered caching, so the builders would be able to use local cache on local SSDs if available, but fall back to copying cache from S3 if not available locally.