Implement simple GitHub CI (#420)

Provides a simple GitHub Actions powered CI, that builds on every PR
and merge to main, and provides downloadable repository zips for
testing or deployment.

.github/workflows/build.yml

@@ -0,0 +1,24 @@
+name: Java CI
+
+on: [push,pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v3
+        with:
+          java-version: '17'
+          distribution: 'adopt'
+
+      - name: Build with Maven
+        run: mvn --batch-mode --update-snapshots package
+
+      - uses: actions/upload-artifact@v3
+        with:
+          name: repository
+          path: repo/target/repository/