Update letsencrypt.ini to support ECDSA keys

Since we have newer certbot available, it's time to support more modern and safer ECDSA keys instead of RSA.

docker/rootfs/etc/letsencrypt.ini

@@ -2,3 +2,5 @@ text = True
 non-interactive = True
 authenticator = webroot
 webroot-path = /data/letsencrypt-acme-challenge
+key-type = ecdsa
+elliptic-curve = secp384r1