In production environments, it is more likely for people to use the stable versions from the docker hub.
The build parameters were thus removed and the image variables uncommented.
Previously, the URL was "front". However, this is not in line with the default URL of the dev environments, where it is "play.workadventure.localhost".
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.
