Update docker-publish.yml

Adding fix for forks to use a variable for Docker image reference

Source of information: https://docs.github.com/en/actions/use-cases-and-examples/publishing-packages/publishing-docker-images

.github/workflows/docker-publish.yml

@@ -9,6 +9,10 @@ on:
     types:
       - published
 
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
 jobs:
   build-and-push:
     name: Build and Push
@@ -26,7 +30,7 @@ jobs:
         id: docker_meta
         uses: docker/metadata-action@v5
         with:
-          images: ghcr.io/pelican-dev/panel
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           flavor: |
             latest=false
           tags: |