Use a more capable tar implementation

The GitLab CI build environment provides a BusyBox version.

bootstrap.sh

@@ -34,3 +34,9 @@ done
 chroot $rootfs apt-get --purge autoremove -q -y
 
 umount $rootfs/dev/pts
+
+tar -caf $rootfs.tar.gz \
+    --directory $rootfs \
+    --exclude './dev/**' \
+    --numeric-owner \
+    --transform 's,^\./,,' .

build.sh

@@ -18,12 +18,6 @@ docker run --rm \
 IMAGE=$CI_REGISTRY_IMAGE
 STAMP=$(date +%F)
 
-tar -caf rootfs.tar.gz \
-    --directory rootfs \
-    --exclude './dev/**' \
-    --numeric-owner \
-    --transform 's,^\./,,' .
-
 cat > Dockerfile <<EOF
 FROM scratch
 ADD rootfs.tar.gz /