fix: allow forks to push to registry with label (#86)

.github/workflows/build.yml

@@ -1,6 +1,7 @@
 name: build-ublue
 on:
-  pull_request:
+  pull_request_target:
+    types: [labeled]
     branches:
       - main
     paths-ignore:
@@ -22,6 +23,7 @@ jobs:
   push-ghcr:
     name: Build and push image
     runs-on: ubuntu-22.04
+    if: contains(github.event.pull_request.labels.*.name, 'ok-to-build')
     permissions:
       contents: read
       packages: write