Simple docker mysq init example:

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$ tree
.
├── docker-compose.yml
└── docker-entrypoint-initdb.d
    └── init_my_db.sql

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$ cat docker-entrypoint-initdb.d/init_my_db.sql 
use pfmc;

create table test_table (id int not null auto_increment primary key, value varchar(255));

insert into test_table (value) values ('toto'), ('pipo'), ('bingo');

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$ cat docker-compose.yml 
version: "3.8"

services:
  mysql:
    image: mysql
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    volumes:
      - pfmc-mysql-data:/var/lib/mysql
      - ./docker-entrypoint-initdb.d:/docker-entrypoint-initdb.d
    environment: 
      MYSQL_ROOT_PASSWORD: pfmc123
      MYSQL_DATABASE: pfmc

volumes:
  pfmc-mysql-data:


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$ docker-compose up -d
Creating network "testdockermysql_default" with the default driver
Creating volume "testdockermysql_pfmc-mysql-data" with default driver
Creating testdockermysql_mysql_1 ... done

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$ docker-compose logs mysql 
Attaching to testdockermysql_mysql_1
mysql_1  | 2020-05-25 11:35:22+00:00 [Note] [Entrypoint]: Entrypoint script for MySQL Server 8.0.19-1debian9 started.
[... cut ...]
mysql_1  | 2020-05-25 11:35:39+00:00 [Note] [Entrypoint]: /usr/local/bin/docker-entrypoint.sh: running /docker-entrypoint-initdb.d/init_my_db.sql
[... cut ...]
| 2020-05-25T11:35:42.682839Z 0 [System] [MY-010931] [Server] /usr/sbin/mysqld: ready for connections. Version: '8.0.19'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  MySQL Community Server - GPL.
mysql_1  | 2020-05-25T11:35:42.801586Z 0 [System] [MY-011323] [Server] X Plugin ready for connections. Socket: '/var/run/mysqld/mysqlx.sock' bind-address: '::' port: 33060


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$ docker-compose exec mysql mysql -p -D pfmc -e "select * from test_table"
Enter password: 
+----+-------+
| id | value |
+----+-------+
|  1 | toto  |
|  2 | pipo  |
|  3 | bingo |
+----+-------+