For MySQL, PostgreSQL, and LDAP, the Guacamole Docker image can use either a hostname+port provided via environment variables or a Docker link. For guacd, however, this is not done. A Docker link is universally required.
It should be trivial for similar GUACD_HOSTNAME and GUACD_PORT environment variables to be supported. Doing this would close a feature gap, and allow the Docker images to be used in situations where Docker links won't always be appropriate (such as when a cluster of guacd instances exist behind a load balancer).