feat(agent): report isDocker so the web shows a docker pull command

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.

internal/agent/daemon.go

@@ -148,6 +148,7 @@ func (d *Daemon) Register(ctx context.Context) error {
 		VPNMode:            d.vpnMode,
 		VPNServer:          d.vpnServer,
 		FunnelURL:          d.funnelURL,
+		IsDocker:           RunningInDocker(),
 	}
 	if free, total, err := DiskInfo(d.cfg.DownloadDir); err == nil {
 		req.DiskFreeBytes = free