Replace docker hub (#28187)

* replace docker hub with mcr

* replace docker hub with mcr

Author: vseanreesermsft

buildpipeline/DotNet-CoreClr-Trusted-Linux-Crossbuild.json

@@ -473,7 +473,7 @@
       "value": "$(DockerRepository):$(DockerTag)"
     },
     "DockerRepository": {
-      "value": "microsoft/dotnet-buildtools-prereqs"
+      "value": "mcr.microsoft.com/dotnet-buildtools/prereqs"
     },
     "DockerTag": {
       "value": "ubuntu-14.04-cross-0cd4667-20170319080304",

buildpipeline/DotNet-CoreClr-Trusted-Linux.json

@@ -433,7 +433,7 @@
       "value": "$(DockerRepository):$(DockerTag)"
     },
     "DockerRepository": {
-      "value": "microsoft/dotnet-buildtools-prereqs"
+      "value": "mcr.microsoft.com/dotnet-buildtools/prereqs"
     },
     "DockerTag": {
       "value": "centos-7-d485f41-20173404063424",

tests/scripts/arm32_ci_script.sh

@@ -377,7 +377,7 @@ function run_tests_using_docker {
             __dockerEnvironmentVariables=" -e ROOTFS_DIR=/crossrootfs/arm"
         ;;
         xenial)
-            __dockerImage=" microsoft/dotnet-buildtools-prereqs:ubuntu1604_cross_prereqs_v3"
+            __dockerImage=" mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu1604_cross_prereqs_v3"
             __skipRootFS=1
             __dockerEnvironmentVariables=" -e ROOTFS_DIR=/crossrootfs/arm"
         ;;