Splitting out from Bug #1801388

If a multi-stage Dockerfile utilizes build ARGS in FROM statements, these are not correctly processed by buildah.

Steps to Reproduce:

1. Create a BuildConfig as follows:
```
apiVersion: build.openshift.io/v1
kind: BuildConfig
metadata:
  name: sample-build-docker-args-preset
spec:
  source:
    dockerfile: |-
      FROM centos
      ARG foo=centos
      FROM $foo
    type: Dockerfile
  strategy:
    dockerStrategy: {}
    type: Docker
```
2. Start the build

Actual Results:

Build fails with error:
```
I0211 22:06:43.140914       1 builder.go:318] Starting Docker build from build config sample-build-docker-args-preset-17 ...
F0211 22:06:43.209605       1 helpers.go:114] error: build error: error parsing name "$foo": error parsing image name "$foo": invalid reference format
```

Expected results:

Build succeeds

Additional Info:

https://github.com/openshift/builder/pull/130#discussion_r377979342