Fix missing envvar escape

[dario-nuro]

Allows new install after cloning.

[maxhelias]

Using ${SERVER_NAME} substitutes the value immediately in Docker Compose, while $${SERVER_NAME} delays substitution until runtime inside the container. In your case, the final value by default will be : https://localhost, php:80/.well-known/mercure and it's not a good value.

[dario-nuro]

Thanks for checking and sorry for rushing into an improper solution, but it allowed me to install.