Cleanup before starting docker TP

Signed-off-by: Arnaud M. <arnaud.choupinette@mailops.fr>

docker/training/lessons/1-docker-getting-started.md

@@ -1,3 +1,11 @@
+# Cleanup your isen
+If necessary, cleanup your `isen` from previous `ansible` TP results:
+
+```bash
+apt-get purge -y nginx
+docker stop demo
+```
+
 # Start the TP
 
 ```bash