Fix Docker toolchain: pin elixir 1.18.5/OTP 27.3.4.17, bump release base to bookworm - #186

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