Dockerfile: Clean-up after apt operations

* Delete old files to keep the image lean.

Dockerfile

@@ -5,7 +5,9 @@ FROM debian:jessie
 
 MAINTAINER Kyle Manna <kyle@kylemanna.com>
 
-RUN apt-get update && apt-get install -y openvpn iptables git-core
+RUN apt-get update && \
+    apt-get install -y openvpn iptables git-core && \
+    apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 # Update checkout to use tags when v3.0 is finally released
 RUN git clone https://github.com/OpenVPN/easy-rsa.git /usr/local/share/easy-rsa