def export(key, val, option: nil)
    string =
      if option == :default
        %{export #{key}="${#{key}:-#{val}}"}
      elsif option == :path
        %{export #{key}="#{val}:$#{key}"}
      else
        %{export #{key}="#{val.gsub('"','\"')}"}
      end

    export = File.join(ROOT_DIR, "export")
    File.open(export, "a") do |file|
      file.puts string
    end
  end
option can be :path, :default, :override
https://github.com/buildpacks/spec/blob/366ac1aa0be59d11010cc21aa06c16d81d8d43e7/buildpack.md#environment-variable-modification-rules