Variable resolution is wrong for ARG variables used in a FROM

[rcjsuen]

The variable resolution code currently always scopes itself to the build stage that the instruction is in. However, in the case of a FROM, it needs to try to resolve itself with the ARG variables that are defined at the beginning of the dockerfile.

import { DockerfileParser } from 'dockerfile-ast';

const content =
`ARG tag=8
FROM node:$tag`

let dockerfile = DockerfileParser.parse(content);
let variables = dockerfile.getAvailableVariables(1);
// this will print out undefined instead of 8
console.log(dockerfile.resolveVariable("tag", 1));