Q:

Why isn't my docker-entrypoint-initdb.d script (as specified in docker-compose.yml) executed to initialize a fresh MySQL instance?

I found out about the /docker-entrypoint-initdb.d directory from this answer, and also read the "Initializing a fresh instance" section of the "How to use this image" MySQL documentation. But when I run docker-compose up in the directory containing the docker-compose.yml file below, my database isn't initialized.

services:

# Use root/root as MySQL user/password credentials
  db:
    image: mysql
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_USER: root
      MYSQL_PASSWORD: root
      MYSQL_DATABASE: db
    volumes:
      - ./mysql/data:/var/lib/mysql
      - ./mysql/init:/docker-entrypoint-initdb.d/:ro

  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080

I confirmed the ./mysql/init directory contains a file named init.sql. And I confirmed that after I empty the ./mysql/data directory and run docker-compose up, that a db database is created. But the database is not populated, unless I manually execute the script in Adminer. (I click on "Import", then choose the file and press the Execute button.)
I looked for messages in the console output after running docker-compose up that indicate an attempt to run init.sql and can't find anything.
Update: The MySQL version is 8.0.19.

A:

Devil hides in details...
You have a double definition of root in your env vars. root user is created by default with password from MYSQL_ROOT_PASSWORD. You then ask to create a second "normal" user... with the exact same name and password (i.e. with MYSQL_USER and MYSQL_PASSWORD)
If you look carefully at your startup log, you will see an error
db_1       | ERROR 1396 (HY000) at line 1: Operation CREATE USER failed for 'root'@'%'

This actually stops further processing of your init files in docker-entrypoint-initdb.d and goes on with the rest of the image startup process (i.e. restarting mysql after initialization on temporary server).
Simply drop MYSQL_USER and MYSQL_PASSWORD in your env vars, or set a different user than root and you will immediately see your init files processed (don't forget to empty your data dir again).

