docs: function needBuild(mode, system, cdef, out, cb) {
      // TODO handle authentication and HTTPS registries
      // also handle other registries than docker-registry-container
  
      var cmds = commands(os.platform());
      var tag = cmds.generateTag(config, system, cdef);
      var baseUrl = 'http://' + config.registry + '/v1/repositories';
      var url = tag.replace(config.registry,
                            baseUrl) + '/tags';
  
      request({ url: url, json: true }, function(err, res, body) {
        if (err) { return cb(err); }
  
        // return the missing definition, or null
        cb(null, Object.keys(body).length === 0 ? cdef : null);
      });
    }