Allow Buildpack Builder Image Override Via app.json

[jamesward]

Proposed app.json schema:

"build": {
    "buildpacks": {
        "builderimage": "heroku/buildpacks"
    }
}

[ahmetb]

Prob can shorten builderimage as "builder" if all builders are going to be images. Other than that I think this is good.

This field can also help the tool follow buildpacks code path directly.

[jamesward]

good call. And builder is what the pack command uses. :)

[jamesward]

Updated schema:

"build": {
    "buildpacks": {
        "builder": "heroku/buildpacks"
    }
}