Windows release Dockerfiles contain Alpha 14 URLs - Should be updated for Alpha15

Steps to reproduce

• https://github.com/PowerShell/PowerShell/releases - Alpha 15 is available now

• View https://github.com/PowerShell/PowerShell/blob/master/docker/release/nanoserver/Dockerfile

• And https://github.com/PowerShell/PowerShell/blob/master/docker/release/windowsservercore/Dockerfile

Expected behavior

Alpha 15 release URLs

Actual behavior

Alpha 14 release URLS