* Add Alpine APK repository proxy support
- Serve named APK repositories at /apk/{repository}/ with the official
Alpine mirror as the default repository
- Cache v2 APKINDEX.tar.gz and v3 Packages.adb indexes and detached
signatures via the metadata cache, serving stored bytes unchanged so
apk signature verification keeps working
- Cache .apk packages in the shared artifact cache keyed by the full
repository path, since APK filenames do not include the architecture
- Add configurable upstream repositories via upstream.apk with
validation, plus dashboard registry instructions
- Add tests for index/signature byte fidelity, per-arch caching, cache
hits, offline reads, upstream authentication, and 404 handling
- Document apk usage in README, config example, and configuration docs
* Serve APK package HEAD requests without a body
Use the method-aware serveArtifact helper (as container.go does) so HEAD
responses carry Content-Length but omit the body; add a regression test.
* Drop doubled blank line in docs/configuration.md
---------
Co-authored-by: Andrew Nesbitt <andrewnez@gmail.com>
