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

@@ -1,3 +1,5 @@
 text = True
 non-interactive = True
 webroot-path = /data/letsencrypt-acme-challenge
+key-type = ecdsa
+elliptic-curve = secp384r1