sec-fix: bump Go toolchain 1.25.12 -> 1.25.13 to close stdlib CVEs - #89

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, docker/build-push-action@v6, docker/setup-buildx-action@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

UndefinedVar: Usage of undefined variable '$PKG_CONFIG_PATH' More info: https://docs.docker.com/go/dockerfile/rule/undefined-var/