Hi @nedaura,
The docker image container for mattermost webrtc only have janus and coturn installed on it to make it a little bit more easy, but you can have a box and install janus for the WebRTC gateway and coturn if you need a TURN server and you may use the configuration files found in the docker container as a reference. Sadly Mattermost WebRTC feature only works with janus as the gateway but the TURN server can be any server with TURN capabilities.
To make it work with an existing install is pretty easy just head to the System Console enable it and add your settings for you webrtc gateway server, then each user has to enable the feature under Account Settings -> Advance -> pre-release features.
WebRTC should work with browsers like Chrome, Firefox I know Safari does not support webrtc capabilities yet. Our desktop apps support the feature in every OS where ir runs.
So far Im not aware of any deb package for this.
Hope this helps