...
elasticsearch:
    image: docker.elastic.co/elasticsearch/elasticsearch:${ELK_VERSION}
    container_name: elasticsearch
    environment:
      - cluster.name=docker-cluster
      - node.name=elasticsearch
      - discovery.seed_hosts=elasticsearch,es02
      - discovery.zen.minimum_master_nodes=1

      - bootstrap.memory_lock=true
      - http.cors.enabled=true
      - http.cors.allow-origin=*
      - ES_JAVA_OPTS=-Xms512m -Xmx512m
      - ELASTIC_PASSWORD=changeme
    ulimits:
      memlock:
        soft: -1
        hard: -1
    volumes:
      - esdata01:/usr/share/elasticsearch/data
    ports:
      - 9201:9200

    networks:
      - elk
...

PUT /_snapshot/my_backup
{
"type": "fs",
"settings": {
"location": "/backup/my_backup"
}
}

{
"error": "RepositoryException[[es_backup_basic] failed to create repository]; nested: CreationException[Guice creation errors:nn1) Error injecting constructor, org.elasticsearch.repositories.RepositoryException: [my_backup] location [/backup/my_backup] doesn't match any of the locations specified by path.repo because this setting is emptyn at org.elasticsearch.repositories.fs.FsRepository.(Unknown Source)n while locating org.elasticsearch.repositories.fs.FsRepositoryn while locating org.elasticsearch.repositories.Repositorynn1 error]; nested: RepositoryException[[es_backup_basic] location [/backup/my_backup] doesn't match any of the locations specified by path.repo because this setting is empty]; ",
"status": 500
}