Ensure /etc/resolv.conf is owned by root:root in the SquashFS.

lb_chroot_resolv will "cp -a" it from the source tree, so it inherits its
ownership from the whoever cloned the Git repository. This has two problems.
First, this results in unsafe permissions on this file (e.g. a Vagrant build
results in the 'amnesia' user having write access to it). Second, building with
a different user results in a non-deterministic SquashFS.

refs: #5630