File ownership not considered when building chroots

[jshuping]

Nomad version

Nomad v0.5.5

Operating system and Environment details

Ubuntu 16.04 on AWS us-east-1 c3.xlarge

Issue

Inside the buildChroot method, there are some file copy and directory creation operations,
which are not considering file/dir ownership, only file/dir permissions.

Reproduction steps

1. Create file /etc/foo/bar.txt, with perms 0400, owned by user/group bob:bob
2. mkdir /etc/bar, with perms 0550, owned by bob:bob
3. Create and run any simple job using the 'exec' driver (so that buildChroot is called)
   3a. (When data_dir is on a separate filesystem from /): Inspect copied filesystem in alloc dir.
   See that /etc/foo/bar.txt has perms 0400 but is owned by root:root (expected bob:bob).
   See that /etc/bar has perms 0550 but is owned by root:root (expected bob:bob)
   3b. (When data_dir is on same filesystem as /): Inspect hardlinked filesystem in alloc dir.
   Remember that dirs cannot be hardlinked, nomad has to create them.
   See that /etc/bar has perms 0550 but is owned by root:root (expected bob:bob)

It would be nice to preserve owner:group on these items. Thanks!

[github-actions]

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