does the spec for `build` follow the docs

[8de2fdb0]

Based of the docs it seems build and image can be used together to use image as the image tag when runningdocker build. The JSON spec doesn't allow both values together, but the cli does it actually exactly as the docs say.

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