This is split out from matrix-cactus-comments (see 241779b583),
but also heavily inspired by `matrix-static-files`.

The container image has only ever been available for amd64,
so not enabling self-building for the other architectures was a mistake
that orignally landed in:
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2089

Looks like these variables were originally named this way in
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2089

Related to https://github.com/mautrix/discord/issues/95

This probably never even worked anyway and was a leftover copy/paste
from some other role.

The docs (`docs/configuring-playbook-bridge-mautrix-wsproxy.md`) only
talk about `matrix_mautrix_wsproxy_hostname`, which was only used via
Traefik labels. The endpoint exposed via `matrix-nginx-proxy` (`/_matrix/wsproxy`)
hasn't been mentioned anywhere.