Prevent internal DNS routing issues when domain used as hostname

[bderenzo]

This PR adds a validation step to ensure the server's $HOST is not mapped to a loopback address (e.g., 127.0.1.1 or 127.0.0.1) in /etc/hosts.

Problem solved :
Many cloud providers automatically add an entry to /etc/hosts mapping the hostname to 127.0.1.1.
When using a domain as hostname and installing BigBlueButton with Greenlight, this loopback mapping causes internal networking conflicts within Docker containers DNS resolution (due to systemd-resolved default configuration). The Greenlight containers attempt to resolve the BBB API locally instead of via the public IP, leading to 400 Bad Request errors when starting rooms.

Full details of the issue can be found here:
bigbluebutton/greenlight#6174

Many users have likely encountered this issue, often without realizing the cause, for instance:

• Fresh installation not working  greenlight#5801
• "This action could not be completed" error in frontend greenlight#5781
• Start button doesn't work and meeting can't start greenlight#5144 (comment)
• BBB room not starting greenlight#2018
• ...