Using BUILDKIT breaks with certain container registries such as CentOS,
with docker build reporting an error such as

  failed to solve with frontend dockerfile.v0:
  failed to build LLB: failed to load cache key:
  unexpected status code
  https://registry.centos.org/v2/centos/manifests/7

:
  403 Forbidden

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210623142245.307776-3-berrange@redhat.com>
Message-Id: <20210709143005.1554-11-alex.bennee@linaro.org>