Is there a way to specify the license at the command line?

You can use the `PORTAINER_LICENSE_KEY` environment variable.  

For example:

docker run -d -p 8000:8000 -p 9443:9443 --name portainer --restart=always -e "PORTAINER_LICENSE_KEY="<license key here>" -v /var/run/docker.sock:/var/run/docker.sock -v portainer_data:/data portainer/portainer-ee:latest