fix deprecated syntax

Author: johnroyer

dev/docker/Dockerfile

@@ -30,5 +30,6 @@ RUN a2enmod rewrite && \
 RUN mv "$PHP_INI_DIR/php.ini-production" "$PHP_INI_DIR/php.ini" && \
     sed -i 's/memory_limit = 128M/memory_limit = 512M/g' "$PHP_INI_DIR/php.ini"
 
-ENV APACHE_DOCUMENT_ROOT /app/public
+ENV APACHE_DOCUMENT_ROOT="/app/public"
+
 WORKDIR /app