add GitHub workflow for building

Author: thestinger

.github/workflows/build.yml

@@ -0,0 +1,20 @@
+name: Build application
+
+on: [pull_request, push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: 21
+          cache: gradle
+      - name: Build with Gradle
+        run: ./gradlew build --no-daemon