Docker container getting connection refused from postgres container in docker-compose

Your port mapping is meaningless inside the docker network. This is only a mapping to the host system. Inside the network, the container is always available on its native port.

 - SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/dbname

Also note that you don’t need to publish the port to access it from inside the network. Doing so for a database can impose security risks. If you can, you should not publish it. That way, it will be only accessible from inside the docker network.