Properly handle paths with symlink path components

This patch fixes the incorrect handling of paths which contain a
symlink as a path component when copying data from a container.
Essentially, this patch changes the container.Copy() method to
first "resolve" the resource by resolving all of symlinks encountered
in the path relative to the container's rootfs.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

Fixes #5619