Support a global persistent-cache namespace via BUILDKIT_CACHE_MOUNT_NS

[cpuguy83]

Summary

Dalec creates many persistent cache mounts (DNF/TDNF, apt, gomod proxy, Go build, sccache, bazel, user-defined caches:). Cache IDs are namespaced ad-hoc per distro/platform/language, but there's no way to add a global prefix to all of them.

BuildKit's Dockerfile frontend supports BUILDKIT_CACHE_MOUNT_NS (a build-arg) which prefixes RUN --mount=type=cache IDs. It's purely a frontend convention — the solver has no notion of it; the frontend just concatenates the namespace onto each cache mount ID. We should adopt the same convention for a consistent, cross-frontend way to namespace persistent caches.

Motivation

A global cache namespace lets users isolate caches without editing every spec or distro config:

• Multi-tenant / multi-project shared builders where caches must not collide or leak between projects.
• CI lanes that want independent caches (e.g. per-branch or per-pipeline) without disabling caching.
• Reproducing a build with a clean cache by changing the namespace, without nuking the shared cache.

Matching the existing BUILDKIT_CACHE_MOUNT_NS arg keeps the knob familiar to BuildKit/Dockerfile users.

Proposed behavior

When --build-arg BUILDKIT_CACHE_MOUNT_NS=<ns> is set, prefix the ID of every persistent cache mount with <ns>/ (or similar), layered on top of the existing distro/platform/language namespacing. Empty/unset preserves today's behavior.

Covers, at minimum:

• RPM package caches — targets/linux/rpm/distro/distro.go, dnf_install.go
• apt caches — helpers.go (WithMountedAptCache)
• gomod proxy / Go build / sccache / bazel and user caches: — cache.go, generator_gomod.go

Implementation notes

• We already read build-arg: opts via client.BuildOpts().Opts (e.g. DALEC_DISABLE_DIFF_MERGE in frontend/request.go, frontend/gateway.go), so reading build-arg:BUILDKIT_CACHE_MOUNT_NS is straightforward.
• Main work is threading the namespace down to the llb.AsPersistentCacheDir(...) call sites. The existing auto-namespacing helpers are the natural place to apply the prefix.