daemon: container: 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 (using pkg/symlink).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
integration-cli: cp: added symlink-related tests

This patch adds cli integration tests for #5619, which are tests
to ensure that symlinks are kept relative to the container rootfs
(even when a path component).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)