web: persist letsencrypt certificates

Store them in the config volume so they are not regenerated on every
boot.

Closes: https://github.com/jitsi/docker-jitsi-meet/issues/71

docker-compose.yml

@@ -9,6 +9,7 @@ services:
             - '${HTTPS_PORT}:443'
         volumes:
             - ${CONFIG}/web:/config
+            - ${CONFIG}/web/letsencrypt:/etc/letsencrypt
         environment:
             - ENABLE_AUTH
             - ENABLE_GUESTS