attach callback with invoke

Author: bruegth

src/Docker.DotNet.X509/CertificateCredentials.cs

@@ -35,7 +35,7 @@ public override HttpMessageHandler GetHandler(HttpMessageHandler handler)
                 managedHandler.ClientCertificates.Add(_certificate);
             }
 
-            managedHandler.ServerCertificateValidationCallback = ServerCertificateValidationCallback;
+            managedHandler.ServerCertificateValidationCallback += (message, certificate, chain, errors) => ServerCertificateValidationCallback?.Invoke(message, certificate, chain, errors) ?? false;
 
             return managedHandler;
         }