Add Docker layer caching to improve build performance

[chorrell]

Implements GitHub Actions cache backend for Docker layer caching across both workflows.

Changes:

update-current-image.yml:

• Added cache-from: type=gha to both 'Build' (test) and 'Build and push Image' (push) steps
• Added cache-to: type=gha,mode=max to store full layer cache for future reuse

dockerimage.yml:

• Added cache-from: type=gha to 'Build Image' step
• Added cache-to: type=gha,mode=max for layer caching

How it works:

• GitHub Actions caches Docker layers between workflow runs
• Layers that haven't changed are reused, skipping re-build time
• Cache is scoped per workflow and branch
• No external infrastructure or credentials required

Expected Impact:

• 5-15 minutes faster per build when layers are cached
• Particularly effective for the second platform build (arm64/amd64) in the same run
• Significant speedup on subsequent runs of the same Node.js version

Note:

• mode=max stores all intermediate layers, maximizing cache hit rate
• Cache will automatically be evicted by GitHub Actions if it exceeds storage limits