[DOCKER] Fix various cron issues (#1204)

Closes
- https://github.com/jmbannon/ytdl-sub/issues/1200
  - Runs in the background now
- https://github.com/jmbannon/ytdl-sub/issues/1201
  - Was not running as user `abc`, now it is
- https://github.com/jmbannon/ytdl-sub/issues/1202
  - Due to reading stdout and tail of hidden log file. Should only tail from hidden log file now

Will leave crontab as-is if no CRON_SCHEDULE is specified

docker/root/custom-cont-init.d/defaults

@@ -21,6 +21,9 @@ echo "Starting ytdl-sub..."
 chown -R ${PUID:-abc}:${PGID:-abc} \
     /config
 
+# always create empty cron log file on start
+echo "" > "$LOGS_TO_STDOUT"
+
 # set up cron
 if [ "$CRON_SCHEDULE" != "" ] ; then
     [[ ! -e "$CRON_SCRIPT" ]] && \
@@ -51,19 +54,15 @@ if [ "$CRON_SCHEDULE" != "" ] ; then
       echo "Cron enabled with schedule $CRON_SCHEDULE_CLEAN"
 
       if [ "$CRON_RUN_ON_START" = true ] ; then
-        echo "Running cron script on start"
-        . "$CRON_WRAPPER_SCRIPT"
+        echo "Running cron script on start in the background"
+        # ensure it runs as abc to respect puid/guid with delay for tail to start
+        su -s "/bin/bash" -c "sleep 5 && . '$CRON_WRAPPER_SCRIPT'" abc > /dev/null 2>&1 &
       fi
     else
       echo "Error in CRON_SCHEDULE definition, disabling cron."
       exit 1
     fi
 else
-  echo "CRON_SCHEDULE not specified, disabling cron."
-  echo "# min   hour    day     month   weekday command" > /config/crontabs/abc
-  echo "" >> /config/crontabs/abc
+  echo "CRON_SCHEDULE not specified, leaving crontabs as-is. Current configuration in /config/crontabs/abc"
+  cat /config/crontabs/abc
 fi
-
-# always create cron log file, after cron runs
-# on start to not tail it again
-echo "" > "$LOGS_TO_STDOUT"