Integrate with buildkit

[hinshun]

POC buildkit using ipcs as psuedo-local build cache.

1. How do define a "cache-miss". For instance, LLB vertex is cacheable with hash sha256:foobar, it's impossible to know if the IPFS network does not have content for that hash. We either traverse the full network, limit hops to certain depth, set a timeout on fetching the content. On a separate topic, can I race a LLB vertex execution to fetching its cache?

[AkihiroSuda]

Execute op and attempt to pull the cache in parallel, and cancel the op if the cache is found?

[hinshun]

Yes exactly, thanks for commenting @AkihiroSuda. I only just started digging through buildkit's cache code, so I'm not that familiar with it yet. For buildkit's containerd worker, does buildkit cache using containerd's content store through its proxy content store? If so, how does the runc worker cache?