In case anyone else has this question:
This seems to be normal the first time you fire up the Magento container. If you look at the Magento logs, it keeps going:
docker logs magento_magento_1
Keep running this command every so often, until you see:
/opt/bitnami/apache/bin/httpd -f /opt/bitnami/apache/conf/httpd.conf -D FOREGROUND’
At this point, you can open your browser to http://localhost and you’ll see the default Magento storefront.
Open your browser to http://localhost/admin, and you can start configuring your store. Login with user / bitnami1
It will take a few minutes, but eventually you’ll see the Dashboard. If not, look at the logs again.