check for file before going sed on it

rootfs/etc/cont-init.d/00-nextcloud

@@ -12,8 +12,9 @@ PGPASSWORD=$DB_PASSWORD psql -h $DB_HOST -U $DB_USER -d $DB_NAME -c "SELECT EXIS
 if [ $? -ne 1 ]; then
     cd /var/lib/nextcloud
     
+    if [ ! -f /var/lib/nextcloud/config/config.php ]; then
         sed -i -e "s/'installed' => true,/'installed' => false,/g" /var/lib/nextcloud/config/config.php
-    
+    fi
     s6-setuidgid nginx ./occ maintenance:install \
         --database=pgsql \
         --database-host=$DB_HOST \