Fix docker image names for release build (#7726)

tools/releaseBuild/Images/microsoft_powershell_centos7/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:centos7
+FROM mcr.microsoft.com/powershell:centos-7
 LABEL maintainer="PowerShell Team <powershellteam@hotmail.com>"
 
 # Install dependencies and clean up

tools/releaseBuild/Images/microsoft_powershell_ubuntu14.04/Dockerfile

@@ -1,6 +1,6 @@
 # Docker image file that describes an Ubuntu14.04 image with PowerShell installed from Microsoft APT Repo
 
-FROM mcr.microsoft.com/powershell:ubuntu14.04
+FROM mcr.microsoft.com/powershell:ubuntu-14.04
 LABEL maintainer="PowerShell Team <powershellteam@hotmail.com>"
 
 # Install dependencies and clean up

tools/releaseBuild/Images/microsoft_powershell_ubuntu16.04/Dockerfile

@@ -1,6 +1,6 @@
 # Docker image file that describes an Ubuntu16.04 image with PowerShell installed from Microsoft APT Repo
 
-FROM mcr.microsoft.com/powershell:ubuntu16.04
+FROM mcr.microsoft.com/powershell:ubuntu-16.04
 LABEL maintainer="PowerShell Team <powershellteam@hotmail.com>"
 
 # Install dependencies and clean up