Handle double quoted strings in dockerfiles.

This handles dockerfiles like this:

```
FROM "ubuntu"
ADD "somefile" "/"
```

**NOTE** single-quoted strings are not handled. There is not special interpolation rules. You can use an empty string, it's up to the command to validate that (and best I could tell, they do).

closes #2105


The regex should be `"[^"]*?"` since quotations are non-greedy.