fix(php-fpm-nginx): forward nginx logs to stdout/stderr

[sylvesterdamgaard]

Problem

The `php-fpm-nginx` image installs nginx with Debian's stock `/etc/nginx/nginx.conf`, which writes:

```
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log warn;
```

Inside a container these are regular files in the overlay filesystem. They never reach `docker logs` / `kubectl logs`. The standalone `nginx` image already does the symlinks — `php-fpm-nginx` just missed them.

Concrete symptom (encountered while running id on this baseimage):

• `kubectl logs ` shows cbox-init's own startup messages
• No access lines, no warnings, no upstream errors from nginx
• PHP-FPM master output makes it through (catch_workers_output + error_log = /proc/self/fd/2) but nginx is silent

Fix

Symlink the two log files to `/dev/stdout` and `/dev/stderr` at build time:

```dockerfile
RUN mkdir -p /etc/nginx/conf.d /var/log/nginx /run/nginx && \
chown -R www-data:www-data /var/log/nginx /run/nginx && \
ln -sf /dev/stdout /var/log/nginx/access.log && \
ln -sf /dev/stderr /var/log/nginx/error.log
```

The existing nginx config keeps writing to `/var/log/nginx/access.log` and `/var/log/nginx/error.log`; those paths now resolve to the container's stdout/stderr. This is the same convention Docker's official nginx image uses, and is what the standalone `nginx` image in this repo already does.

Applied to all 8 stages: slim/standard/chromium/dev × root/rootless.

Test plan

• Build a fresh `8.5-bookworm` tag locally
• Run a Laravel app, hit a route, verify `docker logs` shows the nginx access line
• Trigger an upstream error (make fpm fail), verify the nginx error appears in `docker logs`
• Confirm rootless variants still start cleanly (write access to `/var/log/nginx/` is via the symlinks, owned by root for /dev/stdout — but the symlink itself is fine)