Drop the labs Dockerfile frontend directive

COPY --parents is no longer labs-only: BuildKit's built-in frontend supports
it, so the directive bought nothing and cost a Docker Hub round trip on every
build. The CI builder is created and destroyed per job, so its content store
never caches the frontend image, and a 502 from Docker Hub failed the cuRobo
build outright.

Verified by building all three images with the directive removed; package
sets are unchanged (base 248, kit-less 229, cuRobo 267).

Author: hujc7

docker/Dockerfile.base

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1.7-labs
 # Copyright (c) 2022-2025, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
 # All rights reserved.
 #

docker/Dockerfile.curobo

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1.7-labs
 # Copyright (c) 2022-2025, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
 # All rights reserved.
 #

docker/Dockerfile.kitless

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1.7-labs
 # Copyright (c) 2022-2026, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
 # All rights reserved.
 #