Building with additional contexts

[PaulRudin]

In my workflow file I have:

      - name: Build and push Docker image
        uses: docker/build-push-action@v6
        with:
          context: "{{defaultContext}}:pingchange"
          build-contexts: |
            rtcommon="{{defaultContext}}:/realtime/rtcommon"
          push: true
          tags: |
           ...

but the additional context fails: Error: buildx failed with: ERROR: failed to build: failed to get build context rtcommon:. -Does this sort of thing require an explicit checkout ? Or is there a way to achieve this ?