If you have an instance of n8n running in a Proxmox LCX created from the community script Proxmox VE Helper-Scripts and want to set / change environment variables you need to edit:
nano /etc/systemd/system/n8n.service
Environment variables like WEBHOOK_URL can be set here (note: setting them in .bashrc etc does not work!)
e.g.
[Unit] Description=n8n [Service] Type=simple Environment="N8N_SECURE_COOKIE=false" Environment="WEBHOOK_URL=https://n8n.example.com" ExecStart=n8n start [Install] WantedBy=multi-user.target