ci: docker: Do not hardcode our image path

Fixes the following CI build error inside of forks:

  buildx failed with: ERROR: failed to build: failed to solve: failed to
  push ghcr.io/blue-nebula/server:warnings: denied: permission_denied:
  The requested installation does not exist.

Fixes: 0777e88 ("Automatically publish Docker images on GitHub")

Author: voidanix

.github/workflows/docker.yml

@@ -17,6 +17,10 @@ on:
         required: false
         default: false
 
+env:
+    REGISTRY: ghcr.io
+    IMAGE_NAME: ${{ github.repository }}/server
+
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
@@ -36,7 +40,7 @@ jobs:
       - name: Log in to the Container registry
         uses: docker/login-action@v2
         with:
-          registry: ghcr.io
+          registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
@@ -45,7 +49,7 @@ jobs:
         uses: docker/metadata-action@v4
         with:
           images: |
-            ghcr.io/blue-nebula/server
+            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           tags: |
             type=semver,pattern={{version}},event=tag
             type=ref,event=branch