In the tips section of our elastic beanstalk setup instructions: https://docs.mattermost.com/install/docker-ebs.html
You can find that you can use the command docker exec -ti mattermost-dev /bin/bash to get a shell on the Mattermost docker instance. Where mattermost-dev is the name of the docker container. You can find the name by doing a docker ps.
You should be able to ssh into the elastic beanstalk instance and run the above commands to get a shell where you can run platform commands.