Will be used in the follow-up commits for implementing "Dockerfile hooks"
(issue 4576)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Close issue 4576

- - -

e.g.,
```bash
buildctl build \
  --frontend dockerfile.v0 \
  --opt hook="$(cat hook.json)"
```
with `hook.json` as follows:
```json
{
  "RUN": {
    "entrypoint": ["/dev/.dfhook/entrypoint"],
    "mounts": [
      {"from": "example.com/hook", "target": "/dev/.dfhook"},
      {"type": "secret", "source": "something", "target": "/etc/something"}
    ]
  }
}
```

This will let the frontend treat `RUN foo` as:
```dockerfile
RUN \
  --mount=from=example.com/hook,target=/dev/.dfhook \
  --mount=type=secret,source=something,target=/etc/something \
  /dev/.dfhook/entrypoint foo
```

`docker history` will still show this as `RUN foo`.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>