buildctl: support exporting SolveResponse as a JSON file

[AkihiroSuda]

e.g.,

$ buildctl build  --frontend dockerfile.v0  --local dockerfile=. --local context=. --output type=image,name=foo --response-file resp.json
$ cat resp.json
{
  "exporterResponse": {
    "containerimage.buildinfo": "eyJmcm9udGVuZCI6ImRvY2tlcmZpbGUudjAiLCJzb3VyY2VzIjpbeyJ0eXBlIjoiZG9ja2VyLWltYWdlIiwicmVmIjoiZG9ja2VyLmlvL2xpYnJhcnkvYWxwaW5lOmxhdGVzdCIsInBpbiI6InNoYTI1Njo0ZWRiZDJiZWI1Zjc4YjEwMTQwMjhmNGZiYjk5ZjMyMzdkOTU2MTEwMGI2ODgxYWFiYmY1YWNjZTJjNGY5NDU0In0seyJ0eXBlIjoiaHR0cCIsInJlZiI6Imh0dHBzOi8vcmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbS9tb2J5L2J1aWxka2l0L3YwLjEwLjEvUkVBRE1FLm1kIiwicGluIjoic2hhMjU2OjZlNGI5NGZjMjcwZTcwOGUxMDY4YmUyOGJkMzU1MWRjNjkxN2E0ZmM1YTYxMjkzZDUxYmIzNmU2Yjc1YzRiNTMifV19",
    "containerimage.config.digest": "sha256:715a5c56573fe9c46812f100bb6f359436a3ee627e521310b6ca2bfea2d837a3",
    "containerimage.descriptor": "eyJtZWRpYVR5cGUiOiJhcHBsaWNhdGlvbi92bmQuZG9ja2VyLmRpc3RyaWJ1dGlvbi5tYW5pZmVzdC52Mitqc29uIiwiZGlnZXN0Ijoic2hhMjU2OmRhNWZiMjhiMjgwZGY1MGExMGQxMTg0ZjJmMTU4ZGE2OGNhMmY5YTZkYjUxY2ZkYzFiNmM0YTk5NGFjNTIyMjMiLCJzaXplIjo3MzZ9",
    "containerimage.digest": "sha256:da5fb28b280df50a10d1184f2f158da68ca2f9a6db51cfdc1b6c4a994ac52223",
    "image.name": "foo"
  }
}

This can be used for inspecting metadata that aren't placed in --metadata-file.
e.g., consumedPin data discussed in #2794 (comment)

[tonistiigi]

This can be used for inspecting metadata that aren't placed in --metadata-file.

I don't understand what's the difference. If a field is useful for the user it should in metadata-file.

[AkihiroSuda]

This can be used for inspecting metadata that aren't placed in --metadata-file.

I don't understand what's the difference. If a field is useful for the user it should in metadata-file.

IIUC, the metadata-file should only contain deterministic information.
#2794 (comment)

Non-deterministic information should be excluded from the metadata-file even when they are useful for the user.

[tonistiigi]

My comment wasn't directly about non-determinism but stability of the BuildInfo type. Is there any field atm that behaves differently from metadata file? For consumedPin, that needs to be discussed in the other proposal.