Improves the determination of the proper parent for exec and file ops.
With file ops, it will only consider inputs and ignore secondary inputs.
This prevents the following case:
```
FROM busybox AS build1
RUN echo foo > /hello
FROM scratch
COPY --from=build1 /hello .
```
Previously, `build1` would be considered the parent of the copy
instruction. Now, copy properly does not have a parent.
If there are multiple file ops and the operations disagree on the
canonical "parent", we give up on trying to find a canonical parent and
assume there is none.
For exec operations, whichever input is associated with the root mount
is considered the primary parent.
For all other operations, the first parent is considered the primary
parent if it exists.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
Jonathan A. Sternberg
5c97696d64