CI: migrate off alpine action to a container - #138

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/