Improve resolution of Dockerfile since used in both Vagrantfile and test/Vagrantfile.

vagrant-common.rb

@@ -1,5 +1,6 @@
 $go_regexp = /FROM golang:(\S*).*?/
-$go_version = File.readlines("build/Dockerfile").first { |line| line.match($go_regexp) }.match($go_regexp).captures.first
+$dockerfile_path = File.expand_path(File.join(File.dirname(__FILE__), 'build', 'Dockerfile'))
+$go_version = File.readlines($dockerfile_path).first { |line| line.match($go_regexp) }.match($go_regexp).captures.first
 if $go_version.nil?
   raise ArgumentError.new("Failed to read Go version from Dockerfile.")
 end