feat: add dockerfile and demo program

Dockerfile

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+FROM php:8.2-cli
+
+WORKDIR /app
+COPY . .
+
+RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+RUN composer install
+
+ENV API_BASE_URL=https://api.buttercms.com/v2
+ENV API_KEY=your_api_key
+
+CMD ["php", "demo/demo.php"]

demo/demo.php

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+<?php
+
+require_once 'vendor/autoload.php';
+
+use ButterCMS\ButterCMS;
+
+$apiKey = getenv('API_KEY');
+if (empty($apiKey)) {
+    die("API_KEY environment variable is not set\n");
+}
+
+$client = new ButterCMS($apiKey);
+
+try {
+    $params = ['preview' => 1];
+    $page = $client->fetchPage('*', 'test-page-1', $params);
+    echo "Page Slug: " . $page->getSlug() . "\n";
+    echo "Page Status: " . $page->getStatus() . "\n";
+    echo "Page Scheduled Date: " . $page->getScheduled() . "\n";
+} catch (Exception $e) {
+    echo "Error fetching page: " . $e->getMessage() . "\n";
+}
+
+try {
+    $postResponse = $client->fetchPost('test-blog-post');
+    $post = $postResponse->getPost();
+    echo "Post Slug: " . $post->getSlug() . "\n";
+    echo "Post Status: " . $post->getStatus() . "\n";
+    echo "Post Scheduled Date: " . $post->getScheduled() . "\n";
+} catch (Exception $e) {
+    echo "Error fetching post: " . $e->getMessage() . "\n";
+}
+
+?>