The binary self-update hard-stops inside a container (internal/upgrade
refuses when /.dockerenv exists), so the web's in-app 'force update' button
is futile for Docker agents. Report whether we run in a container via a new
RunningInDocker() helper (UNARR_DOCKER env baked into the image, /.dockerenv
fallback for podman/containerd) on every register + sync, so the web can
swap the button for a copy-paste 'docker compose pull' command.
