fig will create a docker container with a different name than the one used in the fig.yml file.
I got it working by finding the container name with docker ps and looking at the NAMES column.
Then running the psql command in the running container with docker exec -ti NAME_OF_CONTAINER psql -U YOUR_POSTGRES_USERNAME
Important Note:
docker execruns thepsqlcommand on a running containerdocker runwill start a new container.
Update
fig is now called docker-compose