include docker error reason in sandbox start failure response

adapter/http/fastapi/routers/v1/router.py

@@ -70,9 +70,10 @@ def create_sandbox(
             detail=str(exc),
         ) from exc
     except SandboxStartError as exc:
+        detail = f'{exc}: {exc.reason}' if exc.reason else str(exc)
         raise HTTPException(
             status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
-            detail=str(exc),
+            detail=detail,
         ) from exc
     except SandboxError as exc:
         raise HTTPException(