pod dependent env var

When you create a Pod, you can set dependent environment variables for the containers that run in the Pod. To set dependent environment variables, you can use $(VAR_NAME) in the value of env in the configuration file.
https://kubernetes.io/docs/tasks/inject-data-application/define-interdependent-environment-variables/

Container port…

[…] containerPort does not set the port that will be exposed, it is purely informational and had no effect on Kubernetes networking. It is however good practice to document the port that the container is listening on for the next poor soul that is trying to figure out your system.
https://stackoverflow.com/questions/55741170/container-port-pods-vs-container-port-service

The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. It does not make the ports of the container accessible to the host. To do that, you must use the -p flag to publish a range of ports.
https://stackoverflow.com/questions/35548843/does-ports-on-docker-compose-yml-have-the-same-effect-as-expose-on-dockerfile