Add non-root user to PHP Docker images for security

- Create appuser (UID 1000) in dev and prod Dockerfiles
- Set USER appuser to avoid running as root

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/php/dev/Dockerfile

@@ -25,4 +25,8 @@ RUN docker-php-ext-configure gd --with-freetype --with-jpeg \
 RUN pecl install redis imagick \
     && docker-php-ext-enable redis imagick
 
+RUN groupadd -g 1000 appuser && useradd -u 1000 -g appuser -m appuser
+
 WORKDIR /app
+
+USER appuser

docker/php/prod/Dockerfile

@@ -29,4 +29,8 @@ RUN pecl install redis imagick \
 COPY php.ini /usr/local/etc/php/conf.d/app.ini
 COPY opcache.ini /usr/local/etc/php/conf.d/opcache.ini
 
+RUN groupadd -g 1000 appuser && useradd -u 1000 -g appuser -m appuser
+
 WORKDIR /app
+
+USER appuser