Add Docker layer caching to release workflow

[gkorland]

Parent issue

Part of #550 (CI Pipeline Optimization Analysis)

Problem

The release workflow (release-image.yml) builds the Docker image without layer caching, rebuilding all layers from scratch every time.

Solution

Add GitHub Actions cache backend to the Docker build step:

- uses: docker/build-push-action@v5
  with:
    context: .
    file: ./Dockerfile
    push: true
    tags: ${{ env.TAGS }}
    cache-from: type=gha
    cache-to: type=gha,mode=max

Estimated savings

~2-5 min per release build

Effort

Low