Bump actions/checkout from 4 to 7 - #18

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: docker/build-push-action@v6, docker/login-action@v3, docker/metadata-action@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/