Update SignalR Redis tests to use internal Docker Hub mirror

Docker Hub is prone to rate limiting. To prevent this in dotnet builds we should use our internal container registry.

Author: sebastienros

src/SignalR/server/StackExchangeRedis/test/Docker.cs

@@ -16,7 +16,8 @@ public class Docker
 {
     private static readonly string _exeSuffix = OperatingSystem.IsWindows() ? ".exe" : string.Empty;
 
-    private static readonly string _dockerContainerName = "redisTestContainer";
+    private static readonly string _redisImageName = "dotnetdhmirror-f8bzbjakh8cga6ab.azurecr.io/library/redis:7.4";
+    private static readonly string _dockerContainerName = "redisTestContainer74";
     private static readonly string _dockerMonitorContainerName = _dockerContainerName + "Monitor";
     private static readonly Lazy<Docker> _instance = new Lazy<Docker>(Create);
 
@@ -112,7 +113,7 @@ void Run()
             // use static name 'redisTestContainer' so if the container doesn't get removed we don't keep adding more
             // use redis base docker image
             // 30 second timeout to allow redis image to be downloaded, should be a rare occurrence, only happening when a new version is released
-            RunProcessAndThrowIfFailed(_path, $"run --rm -p 6379:6379 --name {_dockerContainerName} -d redis", "redis", logger, TimeSpan.FromMinutes(1));
+            RunProcessAndThrowIfFailed(_path, $"run --rm -p 6379:6379 --name {_dockerContainerName} -d {_redisImageName}", "redis", logger, TimeSpan.FromMinutes(1));
         }
     }