daemon: container: ensure cp cannot traverse outside container rootfs

This patch fixes the bug that allowed cp to copy files outside of
the containers rootfs, by passing a relative path (such as
../../../../../../../../etc/shadow). This is fixed by first converting
the path to an absolute path (relative to /) and then appending it
to the container's rootfs before continuing.

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