This is needed because in the testnode we have :

  os.environ['erp5_tests_bt5_path'] = ','.join(
    os.path.join(parts_directory, x, 'bt5') for x in repository_id_list)

/reviewed-on nexedi/erp5-doc!2

Add a new bt5 for all images used in slapos documentation.

SlapOS user doc (https://slapos.nexedi.com) is committed in this bt5