Docker reported to Connection pool is full, discarding connection: localhost

Server environment: ubuntu 16.04

Client:
 Version:           18.06.1-ce
 API version:       1.38
 Go version:        go1.10.3
 Git commit:        e68fc7a
 Built:             Tue Aug 21 17:24:56 2018
 OS/Arch:           linux/amd64
 Experimental:      false

Server:
 Engine:
  Version:          18.06.1-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       e68fc7a
  Built:            Tue Aug 21 17:23:21 2018
  OS/Arch:          linux/amd64
  Experimental:     false

version: "2.0"
networks:
  ghost:
services:
  ghost:
    image: ghost
    restart: always
    networks:
      - ghost
    ports:
      - 2368:2368
    depends_on:
      - db
      - nginx
    environment:
      database__client: mysql
      database__connection__host: db
      database__connection__user: root
      database__connection__password: abc123123
      database__connection__database: ghost
    volumes:
      - /data/wwwroot:/var/lib/ghost
    container_name: hzzart_ghost
    links:
      - nginx
      - db
    environment:
      - NODE_ENV=production
  nginx:
    image: nginx:latest
    restart: always
    networks:
      - ghost
    ports:
      - "80:80"
    volumes:
      - /data/wwwroot:/usr/share/nginx/html
      - /data/nginx/nginx.conf:/etc/nginx/nginx.conf:ro
      - /data/nginx/conf.d:/etc/nginx/conf.d
      - /data/wwwlogs:/var/log/nginx
    container_name: hzzart_nginx
  db:
    image: mysql:5.7.23
    restart: always
    networks:
      - ghost
    environment:
      - MYSQL_ROOT_PASSWORD=abc123123
      - MYSQL_DATABASE=ghost
    volumes:
      - /data/mysql:/var/lib/mysql

:Connection pool is full, discarding connection: localhost

what is the problem? How to solve it?