Downgrade docker version for windows to 20.10.7

The windows build failed on version 20.10.9, as that
version isn't available to windows. 20.10.7 is the most
recent availble version: [souce](https://dockermsft.azureedge.net/dockercontainer/DockerMsftIndex.json)

Author: eleanorakh

packer/windows/scripts/install-docker.ps1

@@ -1,7 +1,7 @@
 # Stop script execution when a non-terminating error occurs
 $ErrorActionPreference = "Stop"
 
-$docker_version="20.10.9"
+$docker_version="20.10.7"
 $docker_compose_version="1.29.2"
 
 Write-Output "Upgrading DockerMsftProvider module"