Fix config in php-gd extension

Dockerfile

@@ -12,8 +12,9 @@ RUN apt-get update && apt-get install -y \
 
 # Elgg requirements
 RUN docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/
+RUN docker-php-ext-install gd
 RUN docker-php-ext-install pdo pdo_mysql mysql
-RUN docker-php-ext-install mbstring gd
+RUN docker-php-ext-install mbstring
 
 WORKDIR /var/www/html/