Docker: Make DB port changeable via .env

.env.example

@@ -6,3 +6,4 @@ DB_USERNAME=
 DB_PASSWORD=
 DB_DATABASE=
 DB_ROOTPW=
+DB_PORT=

docker-compose.yml

@@ -16,7 +16,7 @@ services:
   mysql:
       image: mysql:8.0
       ports:
-        - 3306:3306
+        - ${DB_PORT}:3306
       volumes:
         - ./data/db:/var/lib/mysql
       environment: