copy should respect user and with_user statements

[erikh]

Basically, copy always copies as root right now. If we getuid/getgid a username inside the container right before copying it, we can modify the tar data at just the right time, allowing this to happen appropriately.

This is a fair bit of work.

[erikh]

On second thought, this is a problematic solution. instead, let's give the copy call a hash where they can overwrite user/group.

[erikh]

Blocked by moby/moby#21651, so we need to fix that first.

[erikh]

Will be resolved partially by moby/moby#28923 once it's implemented. Might require some more patches from us depending on design requirements.