Deployment previews for forks (#2207)

* Create deploy_preview_forks.yml

* skip original deployment on forks

* add comment to explain the conditional

.github/workflows/deploy_preview.yml

@@ -9,7 +9,10 @@ permissions:
 
 jobs:
   deploy-preview:
-    if: ${{ github.repository == 'primer/react' }}
+    # if the source repository for pull request is a fork, we don't want to run this workflow,
+    # we run deploy_preview_forks.yml which uses pull_request_target instead.
+    # see https://github.com/primer/react/pull/2207 for details
+    if: ${{ github.event.pull_request.head.repo.full_name == 'primer/react' }} # source repository for pull_request
     name: Preview
     uses: primer/.github/.github/workflows/deploy_preview.yml@main
     permissions:

.github/workflows/deploy_preview_forks.yml

@@ -0,0 +1,23 @@
+name: Deploy (fork)
+on:
+  pull_request_target:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+jobs:
+  deploy-preview:
+    if: ${{ github.repository == 'primer/react' }} # target repository for pull_request
+    name: Preview
+    uses: primer/.github/.github/workflows/deploy_preview.yml@main
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
+    with:
+      node_version: 14
+      install: npm ci && cd docs && npm ci && cd ..
+      build: npm run build:docs:preview
+      output_dir: docs/public