Pin the base image and fix runtime user creation - #16

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, docker/build-push-action@v4, docker/setup-buildx-action@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/