Overriding/Externalizing application.properties into docker-compose.yml - spring

Question

There are plenty of answers already but none of them solves my problem, please help!
version: '3.8'
services:
mysql:
image: mysql:latest
environment:
- MYSQL_ROOT_PASSWORD=****
- MYSQL_DATABASE=portfolio
- MYSQL_USER=manager
- MYSQL_PASSWORD=******
volumes:
- mysql-data:/var/lib/mysql
ports:
- "8010:3306"
api:
build: ./api
ports:
- "8005:8080"
environment:
- SPRING_DATASOURCE_URL=jdbc:mysql://mysql:3306/portfolio
- SPRING_DATASOURCE_USERNAME=manager
- SPRING_DATASOURCE_PASSWORD=******
- SECRET_REGISTRATION_KEY=&&&&&&
depends_on:
- mysql
volumes:
mysql-data:
Inside my application.properties:
spring.datasource.url=jdbc:mysql://localhost:3306/portfolio_dev
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.username=manager
spring.datasource.password=$#(&%(#%(&%
secret.registration.key=[RANDOM_KEY]
Expected behavior: environment variable like SPRING_DATASOURCE_URL would override spring.datasource.url in application.properties in Spring Boot project.
Actual behavior: api container with Spring Boot inside does not pick up the environment variable in docker-compose.yml, defaulting to the ones inside application.properties.
Reproduction: I ran docker-compose up --build to make sure the JAR files are rebuilt and the mysql server runs fine.

Answer 1

It is really a weird thing when a developer found his solution 10 minutes after posting his desperate StackOverflow question.
Answer: https://stackoverflow.com/a/64940105/15119843
The Spring Boot container starts up right after the mysql starts working, but not ready for connection, so the SB container could not connect to that one.
EDIT: Add restart: on-failure to the Spring Boot container. It restarts the api whenever a failure (that stops the process) as such occur.