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>
---
 tests/docker/docker.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tests/docker/docker.py b/tests/docker/docker.py
index d28df4c140..e9b3af8bfe 100755
--- a/tests/docker/docker.py
+++ b/tests/docker/docker.py
@@ -228,7 +228,9 @@ class Docker(object):
     def __init__(self):
         self._command = _guess_engine_command()
 
-        if "docker" in self._command and "TRAVIS" not in os.environ:
+        if ("docker" in self._command and
+            "TRAVIS" not in os.environ and
+            "GITLAB_CI" not in os.environ):
             os.environ["DOCKER_BUILDKIT"] = "1"
             self._buildkit = True
         else:
-- 
2.31.1