[build] Wire up SONIC_USE_DOCKER_BUILDKIT for parallel Docker layer builds

[rustiqly]

What I did

Wired up the existing SONIC_USE_DOCKER_BUILDKIT config variable to actually enable Docker BuildKit by exporting DOCKER_BUILDKIT=1.

Why I did it

SONIC_USE_DOCKER_BUILDKIT has existed as a config variable — it's passed to the slave container (Makefile.work line 572) and printed in the build config summary (slave.mk line 428) — but was never wired up to actually set DOCKER_BUILDKIT=1. The variable is a dead config knob.

Docker BuildKit provides:

• Parallel layer builds within each Docker image (independent RUN/COPY steps execute concurrently)
• Better cache management with inline cache metadata
• Improved build output with progress tracking
• Available on Docker 18.09+ (all supported build hosts)

How I did it

• Added the config knob documentation to rules/config (default: off/commented out)
• Added export DOCKER_BUILDKIT := 1 in slave.mk when SONIC_USE_DOCKER_BUILDKIT=y

The change is minimal — just exporting an environment variable that Docker picks up automatically for all docker build commands.

How to verify it

# Enable in rules/config.user:
echo 'SONIC_USE_DOCKER_BUILDKIT = y' >> rules/config.user

# Build and observe BuildKit output format (shows parallel layer execution):
make configure PLATFORM=vs
make target/sonic-vs.img.gz 2>&1 | grep -i buildkit

When combined with Docker cache (SONIC_CONFIG_USE_DOCKER_CACHE), BuildKit enables inline cache metadata for efficient layer reuse across builds.

[rustiqly]

Closing this — after digging into the history, BuildKit is already the default on all active build environments.

Full history of SONIC_USE_DOCKER_BUILDKIT:

Date	PR	Author	What happened
Jun 2019	#3049	@stepanblyschak (Mellanox)	Created — added SONIC_USE_DOCKER_BUILDKIT with proper export DOCKER_BUILDKIT=1. Noted Docker bug moby/moby#38903 (--squash + BuildKit = larger images). Default: off.
Apr 2023	#14405	@k-v1	Added explicit disable — export DOCKER_BUILDKIT=0 in else branch, because Docker 23.0 made BuildKit default and broke image sizes via the squash bug.
May 14, 2025	#19952	@sarcot (Microsoft)	Removed entirely — switched to two-stage builds to work around the squash bug. Removed --squash flag and all BuildKit code. Rationale: 'BuildKit is the default now, this option will have no impact.'
May 20, 2025	#22647	@sarcot	Reverted — staged builds broke something
May 28, 2025	#22733	mssonicbld	Second revert
Jul 24, 2025	#23164	@sarcot	Relanded — staged builds re-applied. BuildKit code removed again. But left the $(info) print line and Makefile.work passthrough as dead code.

Current state: All active build environments use Docker 24.0.2+ (bookworm/trixie slave) or Docker 29.x (host in DooD mode). Docker 23+ has BuildKit enabled by default. Nobody sets DOCKER_BUILDKIT=0 anymore. BuildKit is already active — this PR would be redundant.

The dead variable ($(info) print + Makefile.work passthrough) could be cleaned up, but that's cosmetic.