CI: create custom Docker image for faster builds

Add Dockerfile.ci with PHP 8.3, Composer, and GD extension pre-installed. Update workflow to use this custom image instead of rebuilding environment on each run.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Dockerfile.ci

@@ -0,0 +1,21 @@
+FROM php:8.3-cli
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libfreetype6-dev \
+    libjpeg62-turbo-dev \
+    libpng-dev \
+    unzip \
+    git \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install and configure GD extension
+RUN docker-php-ext-configure gd --with-freetype --with-jpeg \
+    && docker-php-ext-install -j$(nproc) gd
+
+# Install Composer
+RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+
+# Set working directory
+WORKDIR /workspace