Move to floating tags

Author: richlander

.vsts-ci.yml

@@ -106,7 +106,7 @@ extends:
         parameters:
           platform:
             name: 'Managed'
-            container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-3e800f1-20190501005343'
+            container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7'
       - ${{ if or(eq(variables['System.TeamProject'], 'public'), in(variables['Build.Reason'], 'PullRequest')) }}:
         - template: /eng/build.yml@self
           parameters:

.vsts-pr.yml

@@ -87,7 +87,7 @@ stages:
     parameters:
       platform:
         name: 'Managed'
-        container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-3e800f1-20190501005343'
+        container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7'
   - ${{ if or(eq(variables['System.TeamProject'], 'public'), in(variables['Build.Reason'], 'PullRequest')) }}:
     - template: /eng/build-pr.yml
       parameters: