Improve `dockerfile verify`

**Description**

The command `cosign dockerfile verify ./Dockerfile` works great if a Dockerfile contains literal image names but not for anything else.

I'd like the two following methods to be supported
1. resolve from variable
```
ARG IMAGE=myimage:mytag
FROM $IMAGE
```
(ARG or ENV)

2. image in COPY
```
COPY --from=myimage:mytag /etc/passwd /etc/passwd
```

/assign