Bases on older node.js to fix issue.

Due to bug [#66](igrigorik/node-spdyproxy#66)
we need to use another base image.

Node.js 0.10 works, version 0.12 or 4 or 6 don't.

Image size: 210 MB -> 190 MB

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:6-slim
+FROM node:0.10-slim
 
 RUN set -x \
     # Install SPDY Proxy.