There is an Elastic Beanstalk-only configuration file called Dockerrun.aws.json. This file is used to configure single or multi-container deployments on Elastic Beanstalk.
For a single-container deployment, Elastic Beanstalk can work with either a Dockerfile or a Dockerrun file.
For a multi-container deployment, Elastic Beanstalk can only work with a Dockerrun file.
These docs may come in handy.