fix: set missing GHCR_REGISTRY var

Author: joedwards32

.github/workflows/docker-publish.yml

@@ -16,8 +16,8 @@ on:
     branches: [ "main" ]
 
 env:
-  # Use docker.io for Docker Hub if empty
   REGISTRY: docker.io
+  GHCR_REGISTRY: ghcr.io
   # github.repository as <account>/<repo>
   IMAGE_NAME: ${{ github.repository }}
 
@@ -51,7 +51,7 @@ jobs:
         if: github.event_name != 'pull_request'
         uses: docker/login-action@f4ef78c080cd8ba55a85445d5b36e214a81df20a
         with:
-          registry: ghcr.io
+          registry: ${{ env.GHCR_REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}