Need to Add Custom XMPP Hostname to Jitsi Env File

[shymky]

Hello.

We have built our own ARENA and Jitsi server using Docker.
During this process, it was necessary to add the following lines to the Jitsi env file:

XMPP_AUTH_DOMAIN=auth.(JITSI_HOSTNAME)
XMPP_DOMAIN=(JITSI_HOSTNAME)
XMPP_INTERNAL_MUC_DOMAIN=internal-muc.(JITSI_HOSTNAME)
XMPP_MUC_DOMAIN=conference.(JITSI_HOSTNAME)
XMPP_RECORDER_DOMAIN=recorder.(JITSI_HOSTNAME)
XMPP_GUEST_DOMAIN=guest.(JITSI_HOSTNAME)
XMPP_SERVER=xmpp.(JITSI_HOSTNAME)
XMPP_BOSH_URL_BASE=http://xmpp.(JITSI_HOSTNAME):5280

Since the default XMPP hostname for Jitsi differs from our custom Jitsi hostname, it seems necessary to specify the XMPP hostname.

Following the current README results in an error saying "Conference server connection failed!", preventing the sharing of Jitsi video and audio among participants in ARENA scenes.

I believe it would be beneficial to add the above information to the README for users of docker-jitsi-meet.