Fix FrankenPHP Docker example and add local dev tip (#8177)

- Replace WORKDIR with SERVER_ROOT env var to correctly set
  FrankenPHP's document root to /app/webroot (default is /app/public)
- Fix composer install running in wrong directory (webroot/ instead of /app)
- Add tip about bin/cake server --frankenphp for local development

Author: dereuromark

docs/en/installation.md

@@ -389,10 +389,11 @@ FROM dunglas/frankenphp
 # Copy your CakePHP application
 COPY . /app
 
-# Set working directory
-WORKDIR /app/webroot
+# FrankenPHP defaults to /app/public as document root.
+# CakePHP uses webroot/ instead, so override it:
+ENV SERVER_ROOT=/app/webroot
 
-# Install dependencies if needed
+# Install dependencies (composer.json lives in /app)
 RUN composer install --no-dev --optimize-autoloader
 
 # Build and run:
@@ -425,6 +426,14 @@ myapp.local {
 
 :::
 
+::: tip Local Development
+For local development, you can use the built-in CakePHP development server with FrankenPHP support:
+```bash
+bin/cake server --frankenphp
+```
+This requires the `frankenphp` binary to be available in your `PATH`.
+:::
+
 ::: info Why FrankenPHP?
 FrankenPHP combines PHP with Caddy, providing automatic HTTPS, HTTP/3, and modern compression without needing PHP-FPM. Particularly efficient for containerized deployments.
 :::