Add stargz snapshotter to moby/buildkit image

[AkihiroSuda]

#1402 added stargz snapshotter for the test image, but the moby/buildkit image on Docker Hub still lacks stargz plugin.

We can consider four options to add stargz plugin to the moby/buildkit image:

• Option 1: Embed the Go package of stargz plugin into buildkitd
• Option 2: Add stargz plugin as a shared library plugin (.so)
• Option 3: Spawn containerd-stargz-grpc process from buildkitd
• Option 4: Spawn containerd-stargz-grpc process from a supervisor daemon like supervisord

I prefer Option 1 or 2.

cc @ktock @tonistiigi

[tonistiigi]

1

[ktock]

I agree with 1.