Update docker.md

[fluidum]

https://maddy.email/docker/

Should fix following:

It will fail on first startup. Copy TLS certificate to /data/tls/fullchain.pem and key to /data/tls/privkey.pem. Run the server again. Finish DNS configuration (DKIM keys, etc) as described in [tutorials/setting-up/](https://maddy.email/docker/tutorials/setting-up/).

This maddy.email page can’t be found
No webpage was found for the web address: https://maddy.email/docker/tutorials/setting-up/
HTTP ERROR 404

[foxcpp]

Thanks!