Upgrade GitHub Actions to Node.js 24-compatible versions

[jhodapp]

GitHub Actions CI workflows use actions that still run on Node.js 20, which is being deprecated:

• June 2, 2026: Actions forced to Node.js 24 by default
• September 16, 2026: Node.js 20 removed from runners entirely

Affected actions (from CI annotations):

• actions/checkout@v4
• actions/cache@v4
• docker/login-action@v3
• docker/setup-buildx-action@v3
• docker/build-push-action@v5

Resolution: Bump to Node.js 24-compatible versions once available, or set FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true to opt in early.