Add GitHub Actions workflow for pull request builds

Author: imbananko

.github/workflows/master.yml

@@ -0,0 +1,18 @@
+name: On open pull request
+on: [pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: 21
+          distribution: jetbrains
+
+      - name: Run gradle build
+        run: ./gradlew build