Edit the app.yml so that the deployment has a ENV variable called FOO with a value that you choose.
https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/
Apply your modified yaml file and watch again with watch kubectl get pod what happens.
Verify with kubectl describe that your ENV variable is set on the pod.
Navigate to the deployment and get a little surprise.
Try to set your ENV variable from a secret you create.