ci: improve caching (Docker images, WASM build artifacts)

[darioAnongba]

Summary

CI performance could be improved with better caching. Currently:

• Esplora Docker image (~1GB+) is pulled fresh every run
• WASM build happens independently in multiple jobs (Node, Esplora integration, Signet smoke test, Browser)
• Rust compilation is cached via Swatinem/rust-cache but each job caches separately

Potential Improvements

Docker image caching

• Use docker/build-push-action with GitHub Actions cache backend
• Or actions/cache to cache docker save output
• Or use a GitHub Container Registry mirror

WASM build artifact sharing

• Build WASM once in a dedicated job, upload as artifact
• Downstream jobs download the artifact instead of rebuilding
• Saves ~2 min per job that currently runs wasm-pack build

Rust cache optimization

• Share Rust cache across jobs with matching keys
• Consider using sccache for distributed compilation caching

Impact

• Current total CI time: ~4-5 min (parallel)
• Expected improvement: reduce Esplora job from ~3 min to ~1.5 min, reduce redundant WASM builds