ci-cd: cache Docker images to speed up CI

[cusma]

We can consider caching the Docker images and invalidating if any changes to Dockerfile or docker-compose.yaml happens in the PR, for example:

      - name: Cache Docker Layers
        uses: satackey/action-docker-layer-caching@v0.1.1
        continue-on-error: true
        with:
          key: docker-cache-${{ runner.os }}-${{ hashFiles('**/Dockerfile', '**/docker-compose.yaml') }}
          restore-keys: docker-cache-${{ runner.os }}-

[cusma]

We should also add a dependency on the book.toml and theme folders for the image caching.