Dockerfile's getAvailableVariables(number) doesn't consider the initial ARGs

[rcjsuen]

import { DockerfileParser } from 'dockerfile-ast';

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

let dockerfile = DockerfileParser.parse(content);
let variables = dockerfile.getAvailableVariables(1);
console.log(variables.length);

0

Given a Dockerfile that has initial ARG instructions defined to be used in subsequent FROM instructions, calling Dockerfile's getAvailableVariables(number) function on a line with FROM should return the initial ARG instructions' variables.