Centos8 uses docker to install WordPress in wordpress+mysql configuration file_ DB_ Understanding of host

This blog mentioned that I follow docker Official website installation wordpress Problems encountered . Maybe it's centos8 Unique problems . After understanding, I'm sure it's not docker Official website composse Documentation issues . Here's what I'm thinking about compose The understanding of the .

below compose The document is almost the same as the official , It's just volumes It's a little different .

version: '3.3'

services:
   db:
     image: mysql:5.7
     volumes:
       - ./db_data:/var/lib/mysql
     restart: always
     ports:
       - "3307:3306"
     environment:
       MYSQL_ROOT_PASSWORD: somewordpress
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress

   wordpress:
     depends_on:
     # It's the same as here mysql Corresponding .
       - db
     image: wordpress:latest
     ports:
       - "8000:80"
     restart: always
     volumes:
       - ./wordpress:/var/www/html
     environment:
      # there db On behalf of the above mysql Container host 
       WORDPRESS_DB_HOST: db:3306
       WORDPRESS_DB_USER: wordpress
       WORDPRESS_DB_PASSWORD: wordpress
       WORDPRESS_DB_NAME: wordpress
volumes:
    db_data: {
    }
    wordpress:

It mainly depends on WORDPRESS_DB_HOST Field , This field corresponds to wp-config.php Medium DB_HOST field value . There are three ways to fill in this value , But the latter two are only workable , Don't actually do this .
（1） there db Refers to mysql service （ip+ Default 3306 port , If it is anything else, it should be clearly pointed out ）, The following port refers to mysql The corresponding port in the container . If it is 3006 You can also omit the words . But it's better to mysql and wordpress Both indicate , Be clear .
（2） According to the first point , If you change wp-config.php Medium DB_HOST The field values for mysql Container of ip+ Ports can also run , It can be changed into 172.19.0.2:3306.
（3） The second point can also be written as run wordpress Of a container default gateway +mysql Stealth port . According to the figure below , It can be written. 172.19.0.1:3307.
The implementation of the latter two methods requires Mr. into two containers , Then query the corresponding Gataway Variable , And then modify wordpress Corresponding wp-config.php Just watch . This is just a way to run , But don't do it . And after re running the container ,wp-config.php The value of will change back .

The same compose The containers where the files run are all under the same gateway . See the picture below .
（1）wordpress Output inspect Some information

（2）mysql Part of the output information