Oh my bad, didn't read the section. Currently you have to set this then in the database manually. Normally the first created users gets the admin flag. To give other users the admin flag, open a webterminal into the app and run the following command, replacing <USERNAME> accordingly:
PGPASSWORD=${CLOUDRON_POSTGRESQL_PASSWORD} psql -h ${CLOUDRON_POSTGRESQL_HOST} -p ${CLOUDRON_POSTGRESQL_PORT} -U ${CLOUDRON_POSTGRESQL_USERNAME} -d ${CLOUDRON_POSTGRESQL_DATABASE} -c "UPDATE users SET admin=true WHERE username='<USERNAME>'"