Add a 'thumbnailer container'

uses the same method as the cron container, runs every 60 seconds.

fixed an issue with the original cron scripts outputting the same date
over and over.

Closes #1

docker-compose.yml

@@ -46,11 +46,29 @@ services:
       while /bin/true; do
         echo Starting cron
         su -s "/bin/sh" -c "/usr/local/bin/php /var/www/html/cron.php" www-data
-        echo $$(date) - Running cron finished
+        echo \$$(date) - Running cron finished
         sleep 900
       done
       EOF'
 
+  thumbnailer:
+    image: nextcloud:fpm-alpine
+    restart: always
+    volumes:
+      - nextcloud:/var/www/html
+    depends_on:
+      - app
+    entrypoint: |
+      sh -c 'sh -s <<EOF
+      trap "break;exit" SIGHUP SIGINT SIGTERM
+      while /bin/true; do
+        echo Starting thumbnail cron
+        su -s "/bin/sh" -c "/usr/local/bin/php occ preview:pre-generate" www-data
+        echo \$$(date) - Running thumbnail cron finished
+        sleep 60
+      done
+      EOF'
+
   proxy:
     build: ./proxy
     restart: always