Update SignalR Redis tests to use internal Docker Hub mirror

[sebastienros]

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

c.f. https://dev.azure.com/dnceng/internal/_wiki/wikis/DNCEng%20Services%20Wiki/1354/ACR-for-Docker-Hub-mirroring

[Copilot]

Pull Request Overview

This PR updates the SignalR Redis test infrastructure to use an internal Azure Container Registry mirror instead of Docker Hub to avoid rate limiting issues during builds.

• Updates the Redis Docker image source from Docker Hub to an internal Azure Container Registry mirror
• Changes the Docker container name to reflect the specific Redis version being used

[Copilot]

The hardcoded Azure Container Registry URL makes the code less maintainable. Consider extracting this to a configuration value or environment variable to allow easier updates when the registry URL changes.

Suggested change

	private static readonly string _redisImageName = "dotnetdhmirror-f8bzbjakh8cga6ab.azurecr.io/library/redis:7.4";
	private static readonly string _redisImageName = Environment.GetEnvironmentVariable("REDIS_IMAGE_NAME")
	?? "dotnetdhmirror-f8bzbjakh8cga6ab.azurecr.io/library/redis:7.4";

[sebastienros]

I don't think we need this, this is a good suggestion though

[akoeplinger]

maybe also extract the ACR host URL into a separate property

[Copilot]

The container name hardcodes the Redis version '74' which will need manual updates when the Redis version changes. Consider deriving the container name from the image version or using a more generic naming approach.

Suggested change

	private static readonly string _dockerContainerName = "redisTestContainer74";
	private static readonly string _redisVersion = _redisImageName.Split(':')[1]; // Extract version from image name
	private static readonly string _dockerContainerName = $"redisTestContainer{_redisVersion.Replace('.', '_')}"; // Replace '.' with '_' for valid container name

[sebastienros]

I don't think this is worth the complexity