scripts/genrootfs.sh: make sure root login is disabled

(cherry picked from commit 72c7a7a3)

scripts/genrootfs.sh

@@ -39,6 +39,9 @@ ${APK:-apk} fetch --keys-dir "$keys_dir" --no-cache \
 	--repositories-file "$repositories_file" \
 	--stdout --quiet alpine-base | tar -zx -C "$tmp" etc/
 
+# make sure root login is disabled
+sed -i -e 's/^root::/root:!:/' "$tmp"/etc/shadow
+
 branch=edge
 VERSION_ID=$(awk -F= '$1=="VERSION_ID" {print $2}'  "$tmp"/etc/os-release)
 case $VERSION_ID in