Variable in FROM resolves to initial ENV instead of ignoring it

[rcjsuen]

import { DockerfileParser } from 'dockerfile-ast';

const content = "ENV image=alpine\nFROM $image";
let dockerfile = DockerfileParser.parse(content);
console.log(dockerfile.resolveVariable("image", 1));

alpine

FROM instructions can only resolve to variables defined by ARG instructions. The resolver should ignore variables defined by ENV instructions when trying to resolve a variable in a FROM.