added iptables package (required by containernetworking/plugins)

Dockerfile

@@ -1,8 +1,10 @@
 FROM debian:buster-slim
 
+# > cni.go:364] [...] exec: "iptables": executable file not found in $PATH
 RUN apt-get update && apt-get install --yes --no-install-recommends \
     ca-certificates \
-    curl
+    curl \
+    iptables
 
 ARG KUBERNETES_VERSION=v1.17.0
 RUN curl --location https://dl.k8s.io/$KUBERNETES_VERSION/kubernetes-node-linux-amd64.tar.gz \