Improve error message when pulling from a private registry

[vf-jes]

Hi,

When pulling from a non existent repository, the error is :

1) Exception in third-party event subscriber: repository does not exist or no read access
#0 /builds/group/project/vendor/beluga-php/docker-php/src/Endpoint/ImageCreate.php(20): Docker\API\Endpoint\ImageCreate->transformResponseBody()

But when the repository exists and require login, it's just a generic error:

1) Exception in third-party event subscriber: Call to a member function wait() on null
#0 /builds/group/project/vendor/testcontainers/testcontainers/src/Container/GenericContainer.php(305): Testcontainers\Container\GenericContainer->pullImage()

In this case the error message should be repository does not exist or no read access too