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>

.forgejo/workflows/deploy.yml

@@ -10,25 +10,8 @@ jobs:
     name: Deploy to Production
     runs-on: docker
     container:
-      image: php:8.3-cli
+      image: forgejo-ci-php:latest
     steps:
-      - name: Install system dependencies and PHP extensions
-        run: |
-          apt-get update && apt-get install -y --no-install-recommends \
-            libfreetype6-dev \
-            libjpeg62-turbo-dev \
-            libpng-dev \
-            unzip \
-            git \
-            curl \
-          && docker-php-ext-configure gd --with-freetype --with-jpeg \
-          && docker-php-ext-install -j$(nproc) gd \
-          && rm -rf /var/lib/apt/lists/*
-
-      - name: Install Composer
-        run: |
-          curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
-
       - name: Checkout code
         run: |
           git clone --depth 1 --branch main https://forge.studio-variable.com/${{ github.repository }}.git .