Dockerfile's resolveVariable resolve variables across build stages incorrectly

[rcjsuen]

import { DockerfileParser } from 'dockerfile-ast';

const content =
`FROM alpine
ARG var=8000
ENV var2=8001
FROM alpine
EXPOSE $var
EXPOSE $var2`

let dockerfile = DockerfileParser.parse(content);
console.log(dockerfile.resolveVariable("var", 4));
console.log(dockerfile.resolveVariable("var2", 5));

8000
8001

As the two EXPOSE arguments are in another build stage, resolveVariable should return undefined instead of resolving to the values defined by the two variables in the first build stage.

[rcjsuen]

Fixed by 7dee77b.