fix: use fallback registry tags on forks

Author: wietzesuijker

.github/workflows/build.yml

@@ -20,6 +20,7 @@ jobs:
 
     env:
       PUSH_IMAGE: ${{ github.repository == 'EOPF-Explorer/data-pipeline' && github.event_name != 'pull_request' && (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/')) }}
+      IMAGE_REGISTRY: ${{ secrets.OVH_HARBOR_REGISTRY || format('ghcr.io/{0}', github.repository_owner) }}
 
     steps:
     - name: Checkout code
@@ -29,7 +30,7 @@ jobs:
       id: meta
       uses: docker/metadata-action@v5
       with:
-        images: ${{ secrets.OVH_HARBOR_REGISTRY }}/eopf-sentinel-zarr-explorer/data-pipeline
+        images: ${{ env.IMAGE_REGISTRY }}/eopf-sentinel-zarr-explorer/data-pipeline
         tags: |
           type=sha
           type=ref,event=branch