Use volumes for database and set more env variables for the connection

docker-compose.yml

@@ -1,28 +1,33 @@
-version: '2'
+version: '3.1'
+
 services:
-  db:
+  database:
     image: mysql:5.7
-    restart: always
     ports:
       - 3306:3306
     environment:
       MYSQL_ROOT_PASSWORD: wordpress
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress
+    volumes:
+      - mysqldata:/var/lib/mysql
 
   wordpress:
     build: .
     user: wordpress
     depends_on:
       - db
-    links:
-      - db
     ports:
       - 8080:8080
     volumes:
       - ./htdocs:/var/www/html
       - ./php-custom.ini:/usr/local/etc/php/conf.d/custom.ini
     environment:
-      WORDPRESS_DB_HOST: db:3306
+      WORDPRESS_DB_HOST: database:3306
       WORDPRESS_DB_PASSWORD: wordpress
+      WORDPRESS_DB_USER: wordpress
+      WORDPRESS_DB_NAME: wordpress
+
+volumes:
+  mysqldata: