The approach you are describing is the recommended for the Docker container. If you want to configure the Bitnami container, I recommend you to read the README.md file in the GitHub repo. There, you will find useful information about how to configure, persist or upgrade your application.
However, we have just found an issue with the SMTP configuration. We are currently working on resolving it and we will let you know about it as soon as possible. Sorry for any inconvenience.
Once it is fixed, if you want to configure SMTP with Bitnami containers, you just need to set some environment variables containing your SMTP credentials. For more details, please read Bitnami Docker Discourse: SMTP Configuration. The recommended way to do it is setting these variables in the docker-compose.yml file.