Missing GUACD_HOSTNAME or "guacd" link

[gwl7810]

FATAL: Missing GUACD_HOSTNAME or "guacd" link.

Every Guacamole instance needs a corresponding copy of guacd running. To
provide this, you must either:
(a) Explicitly link that container with the link named "guacd".
(b) If not using a Docker container for guacd, explicitly specify the TCP
connection information using the following environment variables:
GUACD_HOSTNAME The hostname or IP address of guacd. If not using a guacd
Docker container and corresponding link, this environment
variable is REQUIRED.
GUACD_PORT The port on which guacd is listening for TCP connections.
This environment variable is optional. If omitted, the
standard guacd port of 4822 will be used.