Include Helix Docker image for test reporting : Follow up from #7383.…

… This means we now depend on the Helix clients to do reporting, and the code path is somewhat different for Docker, so let's test one docker variation

src/Microsoft.DotNet.Helix/Sdk.Tests/Microsoft.DotNet.Helix.Sdk.Tests/HelpersTests.cs

@@ -43,6 +43,14 @@ public void VerifyNonEncodedFowardSlashIsConverted()
         public void FailOnceThenPass()
         {
             string target = Path.Combine(Environment.GetEnvironmentVariable("HELIX_WORKITEM_ROOT") ?? Environment.GetEnvironmentVariable("TEMP"), "my-test-file-123456.snt");
+
+            // If we're inside a Helix Docker work item, GetTempPath() is cleaned every execution, 
+            // but the work item's own directory is not (and is writeable from inside Docker), so use it.
+            if (!string.IsNullOrEmpty(Environment.GetEnvironmentVariable("HELIX_DOCKER_ENTRYPOINT")))
+            {
+                target = Path.Combine(Environment.GetEnvironmentVariable("HELIX_WORKITEM_PAYLOAD"), "my-test-file-123456.snt");
+            }
+
             bool exists = File.Exists(target);
             if (!exists)
             {

tests/UnitTests.proj

@@ -61,6 +61,7 @@
     <HelixTargetQueue Include="Debian.9.Amd64"/>
     <HelixTargetQueue Include="RedHat.7.Amd64"/>
     <HelixTargetQueue Include="Windows.10.Amd64"/>
+    <HelixTargetQueue Include="(Debian.10.Amd64)[email protected]/dotnet-buildtools/prereqs:debian-10-helix-amd64-20210304164434-56c6673"/>
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(HelixAccessToken)' == '' ">
@@ -73,6 +74,7 @@
     <HelixTargetQueue Include="Debian.9.Amd64.Open"/>
     <HelixTargetQueue Include="RedHat.7.Amd64.Open"/>
     <HelixTargetQueue Include="Windows.10.Amd64.Open"/>
+    <HelixTargetQueue Include="(Debian.10.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:debian-10-helix-amd64-20210304164434-56c6673"/>
   </ItemGroup>
 
   <PropertyGroup Condition="!$(HelixTargetQueue.StartsWith('Windows'))">