Add small readme for the Dockerfile

README-dockerfile.md

+You may wander what's this `Dockerfile` for. I have tested the Windows Server 2019 insider 17744 build
+to see if I can run the `test.ps1` script that checks all packer templates with `packer validate` in a Windows container.
+
+```
+docker build -t packervalidate .
+```
+
+If the Docker image can be built then all packer templates have no errors.