Allow overriding TRUSTED_HOSTS and MERCURE_PUBLIC_URL variables

[J3m5]

Modifying the SERVER_NAME to disable HTTPS, as outlined in the API Platform documentation, leads to improper configurations for TRUSTED_HOSTS and MERCURE_PUBLIC_URL when using formats like http://localhost or :80.

This results in values like ^:80|php$ or ^http://localhost|php$ for TRUSTED_HOSTS, and URLs such as https://:80/.well-known/mercure or https://http://localhost/.well-known/mercure for MERCURE_PUBLIC_URL.

This PR adopts "nested interpolation" from the Docker documentation to allow precise overrides, introducing TRUSTED_PROXIES and CADDY_MERCURE_PUBLIC_URL as new, adjustable variables, with room for naming discussions.

[vincentchalamon]

LGTM

[dunglas]

LGTM. Could you update the docs to mention these new variables (the current example is still broken, even with your changes, because the new variables must be explicitly set) and also update Symfony Docker accordingly? https://github.com/dunglas/symfony-docker/blob/main/compose.yaml

[J3m5]

LGTM. Could you update the docs to mention these new variables (the current example is still broken, even with your changes, because the new variables must be explicitly set)

Of course.

In which branch this change will be merged?
I suppose I'll need to handle the documentation with the new variables and without in previous branches, right @dunglas ?
How do I should proceed?

and also update Symfony Docker accordingly? https://github.com/dunglas/symfony-docker/blob/main/compose.yaml

Done: dunglas/symfony-docker#591

[dunglas]

Target the 3.3 branch for the docs. We don't have branches for the distribution.