docs: correct the service code on docker-secrets

docs/content/docs/advanced/docker-secrets.md

@@ -6,6 +6,14 @@ If you want to use Docker secrets to store your Tailscale authkey, you can use t
 
 {{% steps %}}
 
+### Requirements ###
+
+Make sure you have Docker Swarm enabled on your server.
+
+<https://docs.docker.com/engine/swarm/secrets/>
+
+"Docker secrets are only available to swarm services, not to standalone containers. To use this feature, consider adapting your container to run as a service."
+
 ### Add a docker secret
 
 We need to create a docker secret, which we can name `authkey` and store the Tailscale authkey in it. We can do that using the following command:
@@ -30,6 +38,8 @@ services:
       # Address of docker server (access to example.com ports)
       - TSDPROXY_HOSTNAME=192.168.1.1 
       - DOCKER_HOST=unix:///var/run/docker.sock 
+    secrets:
+      - authkey
 
 volumes:
   datadir: