Docker compose linking appears to not work
You need to run it using docker-compose:
docker-compose run web mix ecto.createDocker compose creates linked containers, but the images themselves are not linked. This means that blogphoenix_web is not linked to blogphoenix_db, but when you will run
docker-compose upthe newly created containers "blogphoenix_web_1" and "blogphoenix_db_1" will be linked together.