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.
 
    