Fix REGISTRY_NAMESPACE param

The CI has been updated to install packages from the specified
REGISTRY_NAMESPACE.

.github/workflows/build.yml

@@ -5,6 +5,7 @@ on: [push, pull_request]
 env:
   BASE_IMAGE: ${{ vars.BASE_IMAGE || 'registry.fedoraproject.org/fedora:latest' }}
   COPR_REPO: ${{ vars.COPR_REPO || '@pki/master' }}
+  NAMESPACE: ${{ vars.REGISTRY_NAMESPACE || github.repository_owner }}
 
 jobs:
   build:
@@ -14,6 +15,11 @@ jobs:
       - name: Clone repository
         uses: actions/checkout@v4
 
+      - name: Update Dockerfile
+        run: |
+          # update registry namespace
+          sed -i "s/quay.io\/dogtagpki\//quay.io\/$NAMESPACE\//g" Dockerfile
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3