def copy(from, to, options='-a')
    return unless @cache_base

    return false unless File.exist?(from)
    FileUtils.mkdir_p File.dirname(to)
    command = "cp #{options} #{from}/. #{to}"
    system(command)
    raise "Command failed `#{command}`" unless $?
  end
copy cache contents
@param [String] source directory
@param [String] destination directory