Add pull_request trigger so PRs into main post a status check

.gitea/workflows/build.yaml

@@ -14,8 +14,12 @@ name: Build RPi image
 
 on:
   push:
-    # main is release-protected; CI branches may be pushed freely for testing
+    # main is release-protected; ci/* branches are free scratch space
     branches: [main, 'ci/*']
+  pull_request:
+    # gates merges into main, and provides the commit status that a
+    # "require status checks" branch-protection rule waits for
+    branches: [main]
   workflow_dispatch:
 
 # Never run two builds at once on the same (single) runner.