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


I agree that it works, but the reason it works doesn't make sense. `*` is a greedy modifier, so why on earth does it match in a non-greedy way? Also, Go doesn't use backtracking. It uses finite state automata to match regular expressions (there's not really any other way to ensure linear time for any input and any expression).