Added saving certificates for reusing

[virusandy]

trafficstars

While you have a lot of sites and restart your containers often, you have this error:

Register account Error: { "type": "urn:ietf:params:acme:error:rateLimited", "detail": "Error creating new account :: too many registrations for this IP: see https://letsencrypt.org/docs/rate-limits/", "status": 429 }

This is related with letsencrypt-nginx-proxy-companion v.2 issue discussed here https://github.com/nextcloud/docker/pull/1358

The problem can be fixed literally by adding 1 row to the config - so all certificates will be saved in external volumes and won't be lost after each container stop/restart.