Support validation of Dockerfiles with a UTF-8 BOM

[rcjsuen]

const utils = require("dockerfile-utils");
const content = "FROM alpine";
let problems = utils.validate("FROM alpine");
console.log(problems.length);
problems = utils.validate(Buffer.from(`\ufeff${content}`, "UTF-8").toString());
console.log(problems.length);

0
2

This is presumably not the standard use case as no one has ever reported this issue until recently in rcjsuen/dockerfile-ast#106.