Move repo to GitHub Actions (#72)

We've moved away from Semaphore for most repositories. Let's move this
one as well. It's browning it out old images we rely on.

Author: tombruijn

.github/workflows/validate.yml

@@ -0,0 +1,26 @@
+name: Public config validation
+
+on:
+  push:
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' && github.ref != 'refs/heads/develop' }}
+
+jobs:
+  validate:
+    name: Validate
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.3"
+
+      - name: Validate
+        run: rake validate