Sets the cert chain to prefer ISRG Root X1

docker/rootfs/etc/letsencrypt.ini

@@ -3,3 +3,4 @@ non-interactive = True
 webroot-path = /data/letsencrypt-acme-challenge
 key-type = ecdsa
 elliptic-curve = secp384r1
+preferred-chain = ISRG Root X1