Fix Docker development volumes location for the 20-app.dev.ini file

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With the fix to load app's ini files after all other extensions are loaded (#617), the app-dev.ini file has been renamed 20-app.dev.ini and has been moved to a folder named "app.conf.d".

Although, the fix forgot to update the development environment file compose.override.yaml with the new name and location.

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Without this fix, docker raises this error:

$ docker compose up
[+] Running 1/0
 ✔ Container symfony-docker-php-1  Recreated                                                                                                                                                                                                                                                                                                                                                                                          0.1s
Attaching to php-1
Gracefully stopping... (press Ctrl+C again to force)
Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: error mounting "/tmp/symfony-docker/frankenphp/conf.d/app.dev.ini" to rootfs at "/usr/local/etc/php/conf.d/app.dev.ini": mount /tmp/symfony-docker/frankenphp/conf.d/app.dev.ini:/usr/local/etc/php/conf.d/app.dev.ini (via /proc/self/fd/7), flags: 0x5000: not a directory: unknown: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type

[dunglas]

Thanks!