make workflow more generic

.github/workflows/docker-publish.yml

@@ -1,5 +1,8 @@
 name: Build and Push Docker Image to GHCR
 
+env:
+  OCI_REGISTRY: ghcr.io
+
 on:
   push:
     branches: ["main"]
@@ -20,7 +23,7 @@ jobs:
       - name: Log in to GHCR
         uses: docker/login-action@v3
         with:
-          registry: ghcr.io
+          registry: ${{ OCI_REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
@@ -28,7 +31,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ghcr.io/${{ github.repository }}
+          images: ${{ OCI_REGISTRY }}/${{ github.repository }}
 
       - name: Build and push Docker image
         uses: docker/build-push-action@v6