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/sync.go

@@ -175,6 +175,7 @@ func (sc *SyncClient) buildRequest() SyncRequest {
 		LanIP:       sc.cfg.LanIP,
 		TailscaleIP: sc.cfg.TailscaleIP,
 		CanDelete:   sc.cfg.CanDelete,
+		IsDocker:    RunningInDocker(),
 	}
 	if sc.GetTaskStates != nil {
 		req.Tasks = sc.GetTaskStates()