fix: bump Docker actions to node24 majors (clears Node 20 deprecation, #1)

[magicyoda]

Closes #1.

Problem

GitHub is retiring the Node.js 20 actions runtime (forced to Node 24 on 2026-06-02, Node 20 removed 2026-09-16). This composite action pins majors that all run on node20, so every consumer sees the deprecation annotation:

Step	Was	Runtime
docker/login-action	@v3	node20
docker/setup-qemu-action	@v3	node20
docker/setup-buildx-action	@v3	node20
docker/build-push-action	@v5	node20

Change

Bumped to the current majors, each of which declares using: 'node24' in its action.yml:

• docker/login-action @v3 → @v4
• docker/setup-qemu-action @v3 → @v4
• docker/setup-buildx-action @v3 → @v4
• docker/build-push-action @v5 → @v7

The inputs this composite consumes (context, platforms, file, pull, push, tags, target) are unchanged across these majors, so this is non-breaking for consumers.

Validation

Static check (re-run against the live tags): all four target majors declare using: 'node24' in their action.yml (login-action@v4, setup-qemu-action@v4, setup-buildx-action@v4, build-push-action@v7).

The patched composite was also previously exercised end-to-end on a GitHub-hosted runner (login to GHCR → QEMU → Buildx → build & push, success); the only remaining "Node.js 20 deprecated" annotation came from actions/checkout@v4 in the test harness, not from any step inside this composite.

Note: this supersedes the earlier #2, which was closed by accident; the source fork/branch had been deleted, so it could not be reopened and is recreated here.

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🤖 Authored by Claude Code (Opus 4.8), reviewed and run by @magicyoda.