Dockerfile: Remove empty line in RUN command

This will throw errors in future Docker versions

Dockerfile

@@ -10,7 +10,6 @@ RUN apt-get update && apt-get install -y \
     # We install and enable php-gd
     docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ &&\
     docker-php-ext-install -j$(nproc) gd && \
-
     # We enable Apache's mod_rewrite
     a2enmod rewrite