This reverts commit fd3d48bb6d.

Normally this environment variable gets referred to from `settings.json.docker`,
but we have our own full configuration, which hardcodes `"trustProxy": true`,
thus making this useless.

This has been pointed out here:
fd3d48bb6d (commitcomment-47403097)

The official Etherpad Docker image has no support for sqlite3 databases.

don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used