CI: build: fix variable name

.forgejo/workflows/build.yaml

@@ -36,7 +36,7 @@ jobs:
       - name: Log in to container registry
         uses: actions/podman-login@v1
         with:
-          registry: ${{ vars.REPOSITORY }}
+          registry: "docker://${{ vars.REGISTRY }}"
           username: ${{ secrets.REGISTRY_USER }}
           password: ${{ secrets.REGISTRY_PASSWORD }}
 
@@ -50,6 +50,6 @@ jobs:
 
       - uses: actions/push-to-registry@v2
         with:
-          registry: "docker://${{ vars.REPOSITORY }}"
+          registry: "docker://${{ vars.REGISTRY }}"
           image: treetrail-backend
           tags: latest