Fix: Missing ping tool causes unittests inside Docker to fail.

Signed-off-by: Manuel Peuster <manuel@peuster.de>
diff --git a/Dockerfile b/Dockerfile
index 84fa586..d603a5d 100755
--- a/Dockerfile
+++ b/Dockerfile
@@ -42,6 +42,7 @@
     ansible \
     curl \
     iptables \
+    iputils-ping \
     sudo
 
 # install containernet (using its Ansible playbook)