daemon: *: refactored container resource path generation

This patch is a preventative patch, it fixes possible future
vulnerabilities regarding unsantised paths. Due to several recent
vulnerabilities, wherein the docker daemon could be fooled into
accessing data from the host (rather than a container), this patch
was created to try and mitigate future possible vulnerabilities in
the same vein.

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