clarify why copy.Copy is unsafe

[errordeveloper]

The documentation currently states this:

fsutil/copy/copy.go

Lines 65 to 66 in 9e7a6df

	// Copy copies files using `cp -a` semantics.
	// Copy is likely unsafe to be used in non-containerized environments.

It's not clear what is the safety concern exactly.

[tonistiigi]

I think this comment is old, before the proper rootPath protection were added. Before that, the concern was that copying paths that contain symlinks could potentially access files outside of the src/dest arguments.