chore(ci): enable BuildKit cache in Docker workflow

[getsmatu]

Summary

Enable GitHub Actions BuildKit cache for the nethermind Docker build to
reduce build times and improve CI performance.

Details

• Updated .github/workflows/docker.yml:
  • Added cache-from: type=gha and cache-to: type=gha,mode=max to the
    docker/build-push-action step for the nethermind image.

According to Docker's cache management docs, using the GitHub Actions cache
backend for BuildKit allows reusing previously built layers between runs,
which speeds up subsequent builds.

Testing

The workflow still builds and pushes the nethermind image; behavior is
unchanged except for improved caching.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1