Add dockerfile_node_version helper

Author: chorrell

spec/spec_helper.rb

@@ -1,4 +1,14 @@
 module Helpers
+  def dockerfile_node_version(dockerfile_path)
+    File.open(dockerfile_path) do |file|
+      file.each_line do |line|
+        if line =~ /^ENV NODE_VERSION=([0-9]+\.[0-9]+\.[0-9]+)/
+          return $1
+        end
+      end
+    end
+    raise "NODE_VERSION not found in #{dockerfile_path}"
+  end
   def create_image(version)
 
     puts "Building image..."