add github pr workflow (#27)

.github/workflows/build-pull-request.yml

@@ -0,0 +1,25 @@
+# Used when building external pull requests
+# We don't want to publish build artifacts or expose our other caches to possibly untrusted code
+name: build-pull-request
+
+on: [ pull_request ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: gradle/actions/wrapper-validation@v3
+      - uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+        with:
+          cache-read-only: true
+
+      - name: Execute Gradle build
+        run: ./gradlew build