After recently updating my matrix-docker-ansible-deploy installation, matrix-appservice-discord would refuse to start, logging ECONNREFUSED to https://matrix.[mydomain]:443, which was resolving to 172.18.0.2 due to the `--hostname` in mailer grabbing that hostname.

Curious why the IRC bridge didn't have this issue, I looked into it, and it was connecting to `http://matrix-synapse:8008`.  Correcting this one to that URL resolved the issue.