add shell rake task to volume mount fixture in Docker container

Rakefile

+desc "docker shell for fixture"
+task :shell, [:fixture] do |t, args|
+  require_relative "spec/support/buildpack_builder"
+  require_relative "spec/support/path_helper"
+
+  include PathHelper
+  BuildpackBuilder.new(@debug)
+  fixture_path = File.expand_path(fixtures_path(args[:fixture]))
+  system("docker run -i -v #{fixture_path}:/src -t #{BuildpackBuilder::TAG} \"bash\"")
+end