Error parsing complex Dockerfile FROM statement.

```
ARG DOCKER_REGISTRY
FROM ${DOCKER_REGISTRY:+${DOCKER_REGISTRY}/}python:3.10-buster
...
```
Build results in:
```
getting hash for artifact "dsp-test": getting dependencies for "dsp-test": parsing ONBUILD instructions: retrieving image "${DOCKER_REGISTRY:/}python:3.10-buster": parsing reference "${DOCKER_REGISTRY:/}python:3.10-buster": could not parse reference: ${DOCKER_REGISTRY:/}python:3.10-buster
```

### Information

- Skaffold version: v2.6.3
- Operating system: ArchLinux
- Installed via: pacman
- Contents of skaffold.yaml:

```yaml
apiVersion: skaffold/v4beta6
kind: Config
metadata:
name: dsp

build:
artifacts:
- context: ../dsp-test
image: dsp-test
local:
push: false

```

### Steps to reproduce the behavior
`skaffold build`