Use Alpine 3.12 for building PowerShell for alpine (#16008)

tools/releaseBuild/Images/microsoft_powershell_alpine3/Dockerfile

@@ -1,6 +1,6 @@
 # Docker image file that describes an Centos7 image with PowerShell installed from Microsoft YUM Repo
 
-FROM mcr.microsoft.com/powershell:alpine-3.8
+FROM mcr.microsoft.com/powershell:alpine-3.12
 LABEL maintainer="PowerShell Team <powershellteam@hotmail.com>"
 
 # Install dependencies and clean up