In the new version, a new FRONT_URL environment variable is now compulsory in the Pusher.
This is done to setup CORS security in the pusher.
This commit adds this environment variable in the sample docker-compose.prod.yml file.
API_URL is still accepted for BC compatibility of the self-hosted deployments.
This will allow deploying on a single domain, using a reverse proxy that acts on the path.
This however means that the config needs to be changed for all WorkAdventure deployments.
