does the spec for `build` follow the docs

Based of the [docs](https://containers.dev/implementors/json_reference/#image-specific) it seems `build` and `image` can be used together to use `image` as the image tag when running`docker build`. The JSON spec doesn't allow both values together, but the cli does it actually exactly as the docs say.

```json
{
...
"image": "my-devimage",
"build": {
"dockerfile": "Dockerfile",
}
}
```