cleanup PHP sessions

Fixes #14

Dockerfile

@@ -43,7 +43,9 @@ RUN a2enmod rewrite
 RUN crudini --set /etc/php/7.0/apache2/php.ini PHP upload_max_filesize 500M && \
     crudini --set /etc/php/7.0/apache2/php.ini PHP post_max_size 500M && \
     crudini --set /etc/php/7.0/apache2/php.ini PHP max_input_vars 1800 && \
-    crudini --set /etc/php/7.0/apache2/php.ini Session session.save_path /run/wordpress/sessions
+    crudini --set /etc/php/7.0/apache2/php.ini Session session.save_path /run/wordpress/sessions && \
+    crudini --set /etc/php/7.0/apache2/php.ini Session session.gc_probability 1 && \
+    crudini --set /etc/php/7.0/apache2/php.ini Session session.gc_divisor 100
 
 ADD start.sh /app/code/start.sh