Fix random DNS error in nginx due container startup order

[Perteghella]

Problem
Due to the startup order of Docker containers, nginx may start before guacamole. In this case, Nginx tries to resolve the guacamole hostname but fails, and instead uses a gateway IP as the upstream target.

Impact
This results in a "randomly working" Guacamole instance, depending if nginx container start after or befor guacamole.

Solution
Add a depends_on entry for nginx pointing to guacamole (and optionally guacd/postgres) so that Nginx starts only after the required services are available. A more robust solution would also include health checks to ensure guacamole is ready before Nginx starts.