cp's -T and --reflink=auto are only available on GNU coreutils

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Since you know in advance how many strings you need (3 or 4), I would set the capacity to 4: `argv := make([]string, 0, 4)`. Prevents unnecessary allocations.