Only default to logging out to tty if tty exists as a character device

salt/common/tools/sbin/so-image-common

@@ -105,7 +105,11 @@ update_docker_containers() {
   fi
 
   if [ -z "$LOG_FILE" ]; then
-    LOG_FILE=/dev/tty
+    if [ -c /dev/tty ]; then
+      LOG_FILE=/dev/tty
+    else
+      LOG_FILE=/dev/null
+    fi
   fi
 
   # Recheck the version for scenarios were the VERSION wasn't known before this script was imported