Remove hardcoded references to the registry

.forgejo/workflows/build.yaml

@@ -1,7 +1,6 @@
 on: [push]
 env:
-  REGISTRY: forge.hornwitser.no
-  REGISTRY_IMAGE: forge.hornwitser.no/hornwitser/builder:${{ github.ref_name }}
+  REGISTRY_IMAGE: ${{ vars.REGISTRY }}/${{ github.repository }}:${{ github.ref_name }}
 
 jobs:
   build:
@@ -33,7 +32,7 @@ jobs:
       -
         name: Authenticate with registry
         run: |
-          echo "${{ secrets.REGISTRY_TOKEN }}" | buildah login ${{ env.REGISTRY }} --username runner --password-stdin
+          echo "${{ secrets.REGISTRY_TOKEN }}" | buildah login ${{ vars.REGISTRY }} --username runner --password-stdin
       -
         name: Build and push
         run: |