feat: support mainnet and publish Docker Hub images - #23

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, Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/