Support BUILDKIT_CACHE_MOUNT_NS for persistent caches

[kartikjoshi21]

Adds support for the Dockerfile-compatible BUILDKIT_CACHE_MOUNT_NS build arg.
When set, DALEC prefixes persistent BuildKit cache mount IDs with the provided namespace. This gives users a single global knob to isolate cache mounts across projects, CI lanes, branches, or shared builders without editing every spec cache key.

Fixes #1127

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[Copilot]

Pull request overview

Adds BUILDKIT_CACHE_MOUNT_NS support across Dalec’s persistent BuildKit caches.

Changes:

• Threads the global namespace through package, language, and user-defined caches.
• Centralizes cache ID construction and target cache identities.
• Adds tests, schema updates, and documentation.

Reviewed changes

Copilot reviewed 43 out of 43 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
website/content/caches.md	Documents global cache namespacing.
targets/windows/handler.go	Adds configurable Windows cache identity.
targets/windows/handle_zip.go	Applies namespace to Windows build caches.
targets/windows/handle_container.go	Uses configured Windows handlers.
targets/plugin/init.go	Separates alternate-target cache identities.
targets/linux/rpm/suse/sles15.go	Defines SLES cache identity.
targets/linux/rpm/rockylinux/v9.go	Defines Rocky 9 cache identity.
targets/linux/rpm/rockylinux/v8.go	Defines Rocky 8 cache identity.
targets/linux/rpm/distro/pkg.go	Passes namespace into RPM builds.
targets/linux/rpm/distro/dnf_install.go	Namespaces DNF cache mounts.
targets/linux/rpm/distro/distro.go	Centralizes RPM cache IDs.
targets/linux/rpm/distro/determinism_test.go	Tests namespaced RPM caches.
targets/linux/rpm/azlinux/azlinux4.go	Defines Azure Linux 4 cache identity.
targets/linux/rpm/azlinux/azlinux3.go	Defines Azure Linux 3 cache identity.
targets/linux/rpm/almalinux/v9.go	Defines AlmaLinux 9 cache identity.
targets/linux/rpm/almalinux/v8.go	Defines AlmaLinux 8 cache identity.
targets/linux/deb/ubuntu/resolute.go	Defines Resolute cache identity.
targets/linux/deb/ubuntu/noble.go	Defines Noble cache identity.
targets/linux/deb/ubuntu/jammy.go	Defines Jammy cache identity.
targets/linux/deb/ubuntu/focal.go	Defines Focal cache identity.
targets/linux/deb/ubuntu/bionic.go	Defines Bionic cache identity.
targets/linux/deb/distro/worker.go	Namespaces worker apt caches.
targets/linux/deb/distro/pkg.go	Passes namespace into DEB builds.
targets/linux/deb/distro/install.go	Namespaces install-time apt caches.
targets/linux/deb/distro/distro.go	Adds configurable DEB cache identity.
targets/linux/deb/distro/container.go	Namespaces container apt caches.
targets/linux/deb/debian/trixie.go	Defines Trixie cache identity.
targets/linux/deb/debian/bullseye.go	Defines Bullseye cache identity.
targets/linux/deb/debian/bookworm.go	Defines Bookworm cache identity.
source.go	Carries the cache namespace in source options.
preprocess.go	Namespaces preprocessing gomod caches.
packaging/linux/rpm/rpmbuild.go	Applies namespace to RPM user caches.
packaging/linux/deb/pkg.go	Applies namespace to DEB user caches.
load.go	Registers the new build argument.
helpers.go	Adds namespaced apt-cache helpers.
generator_gomod.go	Namespaces gomod proxy caches.
frontend/gateway.go	Reads the namespace build argument.
frontend/gateway_test.go	Tests build-argument extraction.
docs/spec.schema.json	Updates generated cache documentation.
cmd/website/main.go	Uses standardized persistent cache IDs.
cache.go	Applies namespaces across cache types.
cache_id.go	Introduces structured cache ID generation.
cache_id_test.go	Tests cache IDs and namespace behavior.

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