Certificate renewal fails if this runs on a machine serving other websites

[wligtenberg]

I have been running the mailserver for about 3 months now, and the letsencrypt certificates expired.
The service to update them runs fine, however, it cannot renew them, because the challenge fails.
This happens because I use the same server to host a couple of websites using an nginx proxy.

I see two solutions:

1. allow to specify the location of your own certificates using:
   https://docker-mailserver.github.io/docker-mailserver/latest/config/security/ssl/#bring-your-own-certificates

  - SSL_CERT_PATH=/tmp/dms/custom-certs/public.crt
  - SSL_KEY_PATH=/tmp/dms/custom-certs/private.key

2. ensure that the certbot docker image has a predictable name, so I can route the traffic towards it in my nginx configuration.

Willing to help either way. :)