What is the correct way to seed a rails app in docker so it can be started repeatedly

I have a rails app with mysql as the DB, that I'm trying to dockerize, so I can eventually deploy it with docker-machine. When I run docker-compose up the first time, it initializes correctly and seeds the database, like it's supposed to. However, after I shut it down, the second time I run docker-compose up, it fails with the following error, because the seed content is already present:

web_1     | ActiveRecord::RecordNotUnique: Mysql2::Error: Duplicate entry 'antun@example.com' for key 'index_users_on_email'

In my docker-compose.yml file, I'm doing the following under the web service:

command: /bin/sh -c "bin/wait-for db:3306 -- rm -f /home/ubuntu/MY_APP/tmp/pids/server.pid && bundle exec rake db:create db:migrate db:seed && rails server puma -p 80"

I'm using the ! method approach in db/seeds.rb, since I understand that's considered a best-practice:

u = User.new(
  {email: "antun@example.com", encrypted_password: "XXXXX", reset_password_token: nil, reset_password_sent_at: nil, remember_created_at: nil, first_name: "Antun", last_name: "LAST_NAME", provider: "facebook", uid: "12345", username: "antun"}
) 
u.save!(validate: false)

I know I could skip the ! in u.save!() but as I said, I understand it's a best practice to use it in the seeds file, so that it fails. My question is: what is the correct, best-practice approach for creating and seeding a DB only on first run with docker-compose? Once it's live, I want to make sure that subsequent updates to the app (which may include migrations/additional seeds for new data types) work smoothly, without erasing data that's in the database.

Here's the full docker-compose.yml file:

docker-compose.yml

    version: '3.1'
    services:
      db:
        image: mysql:5.6
        command: mysqld --character-set-server=utf8 --collation-server=utf8_unicode_ci --init-connect='SET NAMES UTF8;' --innodb-flush-log-at-trx-commit=0
        environment:
          MYSQL_ROOT_PASSWORD: XXXXX
          MYSQL_DATABASE: XXXXX
          MYSQL_USER: XXXXX
          MYSQL_PASSWORD: XXXXX
      web:
        build: .
        command: /bin/sh -c "bin/wait-for db:3306 -- rm -f /home/ubuntu/MY_APP/tmp/pids/server.pid && bundle exec rake db:create db:migrate db:seed && rails server puma -p 80"
        volumes: 
          - $PWD:/MY_APP
        ports:
          - "80:80"
        links:
          - "db:database" 
        env_file:
          - .env.production
        depends_on:
          - db
      worker:
        build: .
        command: /bin/sh -c "bundle exec bin/delayed_job -n 1 --log-dir=/home/ubuntu/MY_APP/shared/log/delayed_job.log --pool='notifications-poller:1' --pool='broadcast,default,elasticsearch,firebase:2' restart && bundle exec shoryuken --logfile '/home/ubuntu/MY_APP/shared/log/shoryuken.log' --config '/home/ubuntu/MY_APP/config/shoryuken_staging.yml' -R"
        links:
          - "db:database"
        volumes:
          - $PWD:/MY_APP
          - '/home/ubuntu/MY_APP/node_modules'
        env_file:
          - .env.production
        depends_on:
          - db