docs: correct the placement constraints `docker service` example
the constraint expression needs to be quoted
add an actual redis container to run so the command line works
@@ -170,7 +170,8 @@ node type label equals queue:
```bash
$ docker service create \
--name redis_2 \
- --constraint node.labels.type == queue
+ --constraint 'node.labels.type == queue' \
+ redis:3.0.6
```
## Related information
