DEVX-155: fix: use /dev/shm/docker as data-root for inner dockerd

The CI container's overlay (38G) is often 100% full, causing the
inner DinD daemon to fail pulling images (no space left on device).
/dev/shm is a 16G tmpfs with plenty of space for molecule test images.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

src/devx/molecule/start_docker.py

@@ -227,6 +227,10 @@ def start_docker_daemon(timeout: int = DEFAULT_TIMEOUT) -> bool:
     os.environ["DOCKER_HOST"] = f"unix://{DOCKER_SOCK}"
 
     # Start local dockerd (requires privileged container)
+    # Use /dev/shm/docker as data root — the CI container's overlay (38G)
+    # is often 100% full, causing image pull failures. /dev/shm is a 16G
+    # tmpfs with plenty of space for molecule test images.
+    docker_data_root = "/dev/shm/docker"  # nosec B108
     log_file = tempfile.NamedTemporaryFile(  # noqa: SIM115
         mode="w", suffix="dockerd.log", delete=False
     )
@@ -236,6 +240,8 @@ def start_docker_daemon(timeout: int = DEFAULT_TIMEOUT) -> bool:
             "dockerd",
             "--storage-driver",
             "vfs",
+            "--data-root",
+            docker_data_root,
             "-H",
             f"unix://{DOCKER_SOCK}",
         ],