Fix Mercure transport configuration on Docker setups

[CocoPoops]

this was causing my docker setup to not properly start the php container because of an upstream change with FrankenPHP and Mercure

Fixes on Docker Setups:

Error: loading initial config: loading new config: loading frankenphp app module: provision frankenphp: failed to provision caddy http: loading http app module: provision http: server srv0: setting up route handlers: route 0: loading handler modules: position 0: loading module 'subroute': provision http.handlers.subroute: setting up subroutes: route 2: loading handler modules: position 2: loading module 'mercure': provision http.handlers.mercure: provision http.handlers.mercure.bolt: "": invalid transport: open bolt.db: permission denied

see dunglas/symfony-docker#870 and dunglas/symfony-docker#871

[jwr1]

Looks good!