I struggled with this for a while and wasn’t having luck with the accepted answer, I finally got it to work by removing the container:
docker-compose rm postgres
And then the volume as well:
docker volume rm myapp_postgres
Then when I did a fresh docker-compose up I saw CREATE ROLE fly by, which I’m assuming is what was missed on the initial up.
The reasons for this are elaborated on here, on the Git repo for the Docker official image for postgres.