We don't always need to install dev dependencies with Yarn

Dockerfile

@@ -14,6 +14,6 @@ RUN curl -sS https://getcomposer.org/installer | php
 COPY resources/php.ini /usr/local/etc/php/
 COPY . /var/www/html/
 RUN php composer.phar install --prefer-dist
-RUN yarn install
+RUN yarn install --prod
 RUN ./node_modules/.bin/grunt
 ENV CONVERT=1

README.md

@@ -39,7 +39,7 @@ When updating from Git, you need to run yarn and Composer again:
 
 ```bash
 git pull
-yarn install
+yarn install --prod
 composer install
 ```