Here is (https://hub.docker.com/r/schickling/mailcatcher/ docker container) you can use for testing mail sending from your lovely application.
add to your docker-compose.yml
mail: image: schickling/mailcatcher ports: - 1080:1080
Do not forget to link this container to your app container, then use SMTP settings:
STMP: server: mail port: 1025
Open your browser at http://host:1080 and enjoy with preview of caught emails.