Doc: update building acrn in docker for V1.6
One known issue :acrn-issue:`4560` during build ACRN HV.
Please refer to  next section to get Docker Image from Docker Hub.
@@ -31,6 +31,10 @@ This tutorial presents two ways to get the Clear Linux Docker image that's neede
Build the Docker Image from Dockerfile
======================================
+.. note::
+
+ One known issue :acrn-issue:`4560` while building ACRN HV.
+ Please refer to "Get the Docker Image from Docker Hub" to get Docker Image for V1.6 release temporally.
#. Download `Dockerfile <https://raw.githubusercontent.com/projectacrn/acrn-hypervisor/master/doc/getting-started/Dockerfile>`_
to your development machine.
