Azure: Switch to dockerhub provider
    
    `registry.fedoraproject.org/f32/fedora-toolbox` image is used to build
    packages on Azure Pipelines.
    
    registry.fedoraproject.org experiences an availability problem and makes
    unstable FreeIPA CI.
    
    Fedora also distributes its official images on https://hub.docker.com/_/fedora.
    `fedora:32` is already used by FreeIPA CI to build the image for tests.
    
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>