Wordpress - cannot change username or password in docker-compose

If I use this inside docker-compose, it works fine:

db_node_domain:
 image: mysql:5.7
 volumes:
   - ./db_data:/var/lib/mysql
 restart: always
 environment:
   MYSQL_ROOT_PASSWORD: mystrongpassword
   MYSQL_DATABASE: wordpress
   MYSQL_USER: wordpress
   MYSQL_PASSWORD: wordpress
 container_name: wp_db
 networks:
   - "proxy-tier"

wordpress:
 depends_on:
   - db_node_domain
 image: wordpress:latest
 ports:
   - "8080:80"
 expose:
   - "8080"
 restart: always
 environment:
   VIRTUAL_HOST: blog.mydomain.com
   LETSENCRYPT_HOST: blog.mydomain.com
   LETSENCRYPT_EMAIL: foo@mydomain.com
   WORDPRESS_DB_HOST: db_node_domain:3306
   WORDPRESS_DB_USER: wordpress
   WORDPRESS_DB_PASSWORD: wordpress
 container_name: wordpress
 networks:
   - "proxy-tier"

I want to use different username and stronger passwords for MYSQL_USER and WORDPRESS_DB_USER but if I make any changes at all in them I get 502 Bad Gateway from nginx. Can I not change these?

Solution

it is because you have already built the database. When you have run the docker-compose up for the fist time, these values have been set:

 environment:
   MYSQL_ROOT_PASSWORD: mystrongpassword
   MYSQL_DATABASE: wordpress
   MYSQL_USER: wordpress
   MYSQL_PASSWORD: wordpress

and this part:

 volumes:
   - ./db_data:/var/lib/mysql

means it is a persistence data and after stopping the container, for the next run, the data will be available and new ones will not be set. So you have two options:

new build
1. remove the data (or maybe image)
2. modify docker-compose.yml file with new password
3. running docker-compose again
manually change the password, etc
1. run the database container
2. login to the container e.g docker exec -it <CONTAINER-ID> /bin/bash
3. change the password, etc
4. logout and commit this new change

and it is obvious that new build is simpler and better option to do.