【簡単に解決】Dockerコンテナ上のWordPressデータベースにアクセスできない！？対処法：「Can’t select database」「Access denied」「Unknown database」

#MySQLのコンテナに入る
> docker exec -it <コンテナ名> bash

#ルートユーザー権限でMySQLの対話モードに入る
root@dab19e479a65:/# mysql -u <ルートユーザー名> -p<ルートユーザーのパスワード>

#対象ユーザーにデータベースへのアクセス権限を付与する
mysql> grant all privileges on *.* to '<ユーザー名>'@'<ホスト名>';

#対象のデータベースを作成する
mysql> create database <データベース名>;

  p-wp:
    image: wordpress:latest 
    container_name: prograshi_wp
    #restart: always
    depends_on: 
     - db
    ports:
     - "10090:80" 
    environment:
      WORDPRESS_DB_HOST: db:3306 
      WORDPRESS_DB_USER: wp_user 
      WORDPRESS_DB_PASSWORD: root 
      WORDPRESS_DB_NAME: prograshi_db 
      WORDPRESS_DEBUG : 1  
    volumes:
       - ./prograshi:/var/www/html
       

> docker ps
CONTAINER ID   IMAGE                          COMMAND                  CREATED        STATUS        PORTS                                     NAMES
db17e4d0fc2b   wordpress:latest               "docker-entrypoint.s…"   42 hours ago   Up 28 hours   0.0.0.0:10090->80/tcp, :::10090->80/tcp   wp
d01dd3b83a41   phpmyadmin/phpmyadmin:latest   "/docker-entrypoint.…"   42 hours ago   Up 28 hours   0.0.0.0:10099->80/tcp, :::10099->80/tcp   phpmyadmin
dab19e479a65   mysql:5.6                      "docker-entrypoint.s…"   42 hours ago   Up 28 hours   3306/tcp                                  wp_mysql

> docker exec -it wp_mysql bash
root@dab19e479a65:/#

root@dab19e479a65:/# mysql --version
mysql  Ver 14.14 Distrib 5.6.51, for Linux (x86_64) using  EditLine wrapper

# mysql -u root -proot

mysql> select user, host from mysql.user;

+---------+-----------+
| user    | host      |
+---------+-----------+
| root    | %         |
| wp_user | %         |
| root    | localhost |
+---------+-----------+
3 rows in set (0.00 sec)

mysql> show grants for 'ユーザー名'@'ホスト名';

mysql> show grants for 'wp_user'@'%';
+--------------------------------------------------------------------------------------------------------+
| Grants for wp_user@%                                                                                   |
+--------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'wp_user'@'%' IDENTIFIED BY PASSWORD '******'                                    |
| GRANT ALL PRIVILEGES ON `wp\_db`.* TO 'wp_user'@'%'                                                    |
+--------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

grant all privileges on *.* to 'ユーザー名'@'ホスト名';

mysql> grant all privileges on *.* to 'wp_user'@'%';
Query OK, 0 rows affected (0.00 sec)


mysql> show grants for 'wp_user'@'%';
+-----------------------------------------------------------------------------------------------------------------+
| Grants for wp_user@%                                                                                            |
+-----------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'wp_user'@'%' IDENTIFIED BY PASSWORD '******'                                    |
| GRANT ALL PRIVILEGES ON `wp\_db`.* TO 'wp_user'@'%'                                                             |
+-----------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

#現在作成されているデータベース一覧を表示
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| wp_db              |
+--------------------+
4 rows in set (0.00 sec)

#データベースを新規作成
mysql> create database prograshi_db;
Query OK, 1 row affected (0.00 sec)

#確認
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| wp_db              |
| prograshi_db       |
+--------------------+
5 rows in set (0.00 sec)