feat(ruby): add Alpine Linux (musl libc) support for x86_64 and aarch64

[prateek-kumar-improving]

Summary

Add Alpine Linux (musl libc) support for the valkey-rb gem on both x86_64 and aarch64 architectures. This enables users running Docker containers based on Alpine to use the gem with
prebuilt native libraries.

Issue link

This Pull Request is linked to issue: #98

Features / Behaviour Changes

• The gem now automatically detects musl libc at runtime and loads the correct native library (*-unknown-linux-musl vs *-unknown-linux-gnu)
• Alpine Linux 3.18+ is now a supported platform for both x86_64 and aarch64
• CI validates the gem on Alpine Linux with full test coverage (standalone, TLS, cluster)
• CD builds and publishes musl-compatible native libraries alongside glibc ones

Implementation

Runtime detection (lib/valkey/bindings.rb):

• Added musl_libc? class method with a 3-check cascade:
  1. RUBY_PLATFORM contains "musl"
  2. /etc/alpine-release file exists
  3. RbConfig::CONFIG["target_os"] contains "musl"
• platform_info now resolves {arch}-unknown-linux-musl when musl is detected

Build matrix (.github/json_matrices/build-matrix.json):

• Added x86_64-unknown-linux-musl and aarch64-unknown-linux-musl target entries

CI workflow (.github/workflows/CI.yml):

• Excluded musl targets from the regular glibc-based build/test jobs
• Added dedicated build-and-test-musl job running natively inside ruby:3.3-alpine container on x64
• Builds Rust FFI library with RUSTFLAGS="-C target-feature=-crt-static" (required for dynamic linking on musl)
• Runs full test suite including standalone, TLS, and cluster

CD workflow (.github/workflows/cd.yml):

• Added musl cross-compilation steps (musl-tools, rustup target, conditional --target flag)
• Adjusted artifact upload paths for target-specific Cargo output directory
• Added test-gem-musl job to verify the published gem loads and passes tests on Alpine

Test fix (test/valkey/uri_connection_test.rb):

• Skip test_ssl_with_unreadable_file when running as root (Alpine CI container), since root bypasses file permission checks

Limitations

• ARM64 (aarch64) musl is built via cross-compilation in CD but cannot be integration-tested in CI containers due to a [GitHub Actions limitation](https://github.blog/changelog/2025-
  09-19-deprecation-of-node-20-on-github-actions-runners/) where JavaScript-based actions don't work in Alpine containers on ARM64 runners. The x64 musl path has full integration
  testing.
• Only Alpine Linux 3.18+ is tested; other musl-based distros (e.g., Void Linux) should work but are untested.

Testing

• Full standalone, TLS, and cluster test suites pass on Alpine Linux (x86_64)
• Semgrep security scanning passes (actions pinned to commit SHAs, no curl-pipe-shell)
• Existing glibc and macOS test suites unaffected

Checklist

• This Pull Request is related to one issue.
• Commit message has a detailed description of what changed and why.
• Tests are added or updated.
• CHANGELOG.md and documentation files are updated.
• Linters have been run (bundle exec rubocop) and pass.
• Destination branch is correct - main
• Create merge commit if merging release branch into main, squash otherwise.
• Make sure to update the documentation in the valkey-glide-docs repository if necessary

[alexr-bq]

Have you ran the CD workflow (in non-publish / dry-run mode) and validated it?

[prateek-kumar-improving]

Have you ran the CD workflow (in non-publish / dry-run mode) and validated it?

The CD workflow worked: https://github.com/valkey-io/valkey-glide-ruby/actions/runs/29279904222

[prateek-kumar-improving]

CD workflow works after the last code change: https://github.com/valkey-io/valkey-glide-ruby/actions/runs/29351834655