Honor user passed on container in nsinit

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

system/calls_linux.go

@@ -71,6 +71,14 @@ func Setresuid(ruid, euid, suid int) error {
 	return syscall.Setresuid(ruid, euid, suid)
 }
 
+func Setgid(gid int) error {
+	return syscall.Setgid(gid)
+}
+
+func Setuid(uid int) error {
+	return syscall.Setuid(uid)
+}
+
 func Sethostname(name string) error {
 	return syscall.Sethostname([]byte(name))
 }