Add function to resolve variables in image references

This release adds a new function to resolve variable substitutions in image references.

For example, given this Dockerfile:

ARG IMAGE=alpine
ARG TAG=3.12
FROM ${IMAGE}:${TAG}

... previously the image would be parsed as:

ImageRef { registry: None, image: "${IMAGE}", tag: Some("${TAG}"), hash: None }

...which is obviously unhelpful. This default behavior remains unchanged, however the new function ImageRef::resolve_vars(&self, &Dockerfile) can be called to resolve these variable references when possible:

let d = Dockerfile::parse(indoc!(r#"
  ARG image=alpine
  ARG unnecessarily_nested=${image}
  ARG tag=3.12
  FROM ${unnecessarily_nested}:${tag}
"#)).unwrap();

let from: &FromInstruction = d.instructions
  .get(3).unwrap()
  .try_into().unwrap();

assert_eq!(
  from.image_parsed.resolve_vars(&d),
  Some(ImageRef::parse("alpine:3.12"))
);

Note that this adds dependencies on regex and lazy_static.