Hi @overmat ,
the SiteURL does not depend on the installation being publicly available or on the fact if you’re using TLS or not. Its value should be exactly what your users will put into their clients and browsers then when they try to access your Mattermost installation, so if your Mattermost container is running on host 192.168.0.1 on port 8065 but you want to proxy it through caddy and caddy is running on 192.168.0.2 on port 80 and you do not use internal hostnames, your SiteURL should be set to http://192.168.0.2.
What error are you seeing when you try to access the web interface locally and what does local mean?