test: update docker preflight assertion for stdin=DEVNULL kwarg

The blanket stdin=subprocess.DEVNULL pass added the kwarg to the docker
'version' preflight call; the test pinned the exact kwargs dict. Update
the expected dict to match.

tests/tools/test_docker_environment.py

@@ -109,6 +109,7 @@ def test_ensure_docker_available_uses_resolved_executable(monkeypatch):
             "capture_output": True,
             "text": True,
             "timeout": 5,
+            "stdin": subprocess.DEVNULL,
         })
     ]