Support multiple arguments in ARG

[rcjsuen]

In Docker Engine 20.10, ARG instructions can now accept multiple arguments.

FROM scratch
ARG a=b b

$ docker build .
Sending build context to Docker daemon     47MB
Step 1/2 : FROM scratch
 ---> 
Step 2/2 : ARG a=b b
 ---> Running in e60e9e9855ec
Removing intermediate container e60e9e9855ec
 ---> c6a7360077c7
Successfully built c6a7360077c7

This code should now print out 0 instead of 1.

const dockerfileUtils = require("dockerfile-utils");
const results = dockerfileUtils.validate("FROM scratch\nARG a=b b");
console.log(results.length);