Removed apt list after install

Dockerfile

@@ -3,8 +3,7 @@
 
 FROM wordpress:4.9.2-fpm
 
-RUN apt-get update \
-	&& apt-get install -y openssh-server ssmtp \
+RUN apt-get update && apt-get install -y openssh-server ssmtp && rm -rf /var/lib/apt/lists/* /tmp/* \
 	&& useradd -c "Hosting Admin User" -u 1000 -g users -G www-data -d /var/www/html -M lamp \
 	&& sed -i -e 's/^mailhub=mail$/mailhub=smtp/' -e "s/^hostname=/#hostname=/" -e 's/#FromLineOverride=YES/FromLineOverride=YES/' /etc/ssmtp/ssmtp.conf