Update env file handling in Docker setup

Changed the environment file renaming to use a variable for better flexibility. Now it moves the sample env file based on the website root path.

Dockerfile

@@ -32,7 +32,7 @@ RUN set -eux; \
 
 USER www-data
 WORKDIR ${WEBSITE_ROOT}
-RUN mv .env.sample .env
+RUN mv ${WEBSITE_ROOT}.env.sample ${WEBSITE_ROOT}.env
 RUN composer install
 RUN php bin/console asset-map:compile