server: add Dockerfile and environment variables

The Dockerfile (and docker-compose) simplify the server's deployment.
The environment variables avoid secrets in the code.

Dockerfile

@@ -0,0 +1,8 @@
+FROM docker.io/library/php:8.3-apache
+
+COPY server/*.php /var/www/html
+COPY server/.htaccess /var/www/html
+
+RUN a2enmod rewrite && docker-php-ext-install pdo pdo_mysql
+
+EXPOSE 80