Installed superset using the docker image, but I was never prompted for an admin account creation. I've tried admin/admin and admin/superset combinations but nothing is working.
You should have been prompted to create an admin account at the end of Docker build.
If you missed it, you can use following commands to create a new admin account:
docker-compose exec superset fabmanager create-admin --app superset
Or
docker-compose exec superset bash -c 'export FLASK_APP=superset && flask fab create-admin'
Make sure you are in the contrib/docker folder and your containers are up and running.