fix: handle fork builds without registry secrets

Author: wietzesuijker

.github/workflows/build.yml

@@ -18,6 +18,10 @@ jobs:
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/') || github.event_name == 'pull_request'
 
+    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
       uses: actions/checkout@v5
@@ -26,7 +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
@@ -38,6 +42,7 @@ jobs:
       uses: docker/setup-buildx-action@v3
 
     - name: Log in to OVH Container Registry
+      if: env.PUSH_IMAGE == 'true'
       uses: docker/login-action@v3
       with:
         registry: ${{ secrets.OVH_HARBOR_REGISTRY }}
@@ -50,7 +55,7 @@ jobs:
         context: .
         file: docker/Dockerfile
         platforms: linux/amd64
-        push: true
+        push: ${{ env.PUSH_IMAGE == 'true' }}
         tags: ${{ steps.meta.outputs.tags }}
         labels: ${{ steps.meta.outputs.labels }}
         cache-from: type=gha