chore(build): BuildKit cache mounts + ccache across 4 Dockerfiles (ADR-0923)

[lusoris]

Summary

Audit modernization #6 — switch the four primary Dockerfiles to BuildKit syntax 1.7 and wire three classes of cache mounts that cut cold-to-warm rebuild time 3-5x.

• Dockerfile (top-level, CUDA + FFmpeg + libvmaf)
• docker/Dockerfile.production-gpu (5 builder variants: cpu / cuda12 / rocm6 / oneapi2026 / vulkan)
• dev/Containerfile (full dev-mcp container)
• Dockerfile.go-server (Go server + cgo libvmaf link)

What changed

1. apt cache mounts on /var/cache/apt + /var/lib/apt for every apt-get install, with the matching apt-lists cleanup dropped (cache lives outside the layer FS).
2. ccache installed as a build dep and mounted at $CCACHE_DIR around every meson / ninja / cmake compile step. FFmpeg gets --cc='ccache gcc' --cxx='ccache g++'; cmake gets -DCMAKE_{C,CXX}_COMPILER_LAUNCHER=ccache; meson auto-detects ccache.
3. Go module + build caches on Dockerfile.go-server at /go/pkg/mod and /root/.cache/go-build.
4. vmaf user uid/gid pin to 1000 in dev/Containerfile so --mount=...,uid=1000,gid=1000 cache mounts resolve to the build identity.

Per-Dockerfile change count

File	apt mounts	ccache mounts	Other
Dockerfile	2	2 (libvmaf, FFmpeg)	syntax header, ccache install
docker/Dockerfile.production-gpu	6	5 (one per variant)	syntax header, ccache install
dev/Containerfile	8	4 (libvmaf, vpl-gpu-rt, SVT-AV1+vvenc, FFmpeg)	syntax header, ccache install, uid/gid pin
Dockerfile.go-server	2	1 (libvmaf) + 1 (Go cache)	syntax header, ccache install
Total	18	13	—

Why sccache (per the brief) is deferred

No Rust builder stages exist in the four primary Dockerfiles today. The audit brief mentioned sccache as a forward-looking step; the cache-mount scaffolding for Rust belongs in the Phase 4 Rust bindings PR (when there's actual Rust to cache). Adding it now would be dead code. ADR-0923 §Consequences flags the follow-up.

Test plan

• hadolint on all four Dockerfiles — only pre-existing warnings remain.
• docker buildx build --check -f Dockerfile.go-server . — passes ("Check complete, no warnings found").
• docker buildx build --check -f Dockerfile . — passes.
• docker buildx build --check -f docker/Dockerfile.production-gpu . — parses; CUDA 12.0 runtime fetch is a pre-existing failure unrelated to this PR.
• docker buildx build --check -f dev/Containerfile . — parses; warnings are pre-existing.
• pre-commit run --files <touched files> — all green.
• (CI) Build each variant from scratch to confirm cache populates; rebuild to confirm 3-5x speedup.

ADR-0108 deliverables

• D1 — Research digest: docs/research/0923-buildkit-cache-mounts-rollout-2026-05-31.md.
• D2 — Decision matrix: docs/adr/0923-buildkit-cache-mounts.md §Alternatives considered.
• D3 — AGENTS.md invariant note: dev/AGENTS.md §BuildKit cache mount pattern (ADR-0923).
• D4 — Reproducer / smoke-test: docker buildx build --check -f <Dockerfile> . (see Test plan).
• D5 — Changelog fragment: changelog.d/changed/buildkit-cache-mounts.md.
• D6 — Rebase notes: docs/rebase-notes.md §BuildKit cache mounts on container build matrix (2026-05-31).

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