How to import a database into a Docker container using docker-compose

I am working with the official WordPress Docker image with docker-compose on my Mac (using boot2docker). I need to do a one-off data import. I'm not sure how to do this. How can I import data to the database container?

wordpress:
  image: wordpress
  links:
    - db:mysql
  ports:
    - 8080:80
  volumes:
    - .:/var/www/html/wp-content/themes/my-theme-name
db:

image: mariadb

environment:

MYSQL_ROOT_PASSWORD: example

#docker