chore: Improve error message when Docker is not running (#987)

Co-authored-by: Andre Hofmeister <9199345+HofmeisterAn@users.noreply.github.com>

src/Testcontainers/Builders/AbstractBuilder`4.cs

@@ -126,7 +126,7 @@ protected virtual TBuilderEntity Init()
     /// <exception cref="ArgumentException">Thrown when a mandatory Docker resource configuration is not set.</exception>
     protected virtual void Validate()
     {
-      const string message = "Cannot detect the Docker endpoint. Use either the environment variables or the ~/.testcontainers.properties file to customize your configuration:\nhttps://dotnet.testcontainers.org/custom_configuration/";
+      const string message = "Docker is either not running or misconfigured. Please ensure that Docker is running and that the endpoint is properly configured. You can customize your configuration using either the environment variables or the ~/.testcontainers.properties file. For more information, visit:\nhttps://dotnet.testcontainers.org/custom_configuration/";
       _ = Guard.Argument(DockerResourceConfiguration.DockerEndpointAuthConfig, nameof(IResourceConfiguration<TCreateResourceEntity>.DockerEndpointAuthConfig))
         .ThrowIf(argument => argument.Value == null, argument => new ArgumentException(message, argument.Name));
     }