Fix #899 -- Docker container: Set gunicorn workers to two times the CPU count

deployment/docker/pretix.bash

@@ -3,7 +3,7 @@ cd /pretix/src
 export DJANGO_SETTINGS_MODULE=production_settings
 export DATA_DIR=/data/
 export HOME=/pretix
-NUM_WORKERS=10
+export NUM_WORKERS=$((2 * $(nproc --all)))
 
 if [ ! -d /data/logs ]; then
     mkdir /data/logs;