Migrate to GitHub Actions

Drop the broken and unused reprotest-sd job entirely, leave a
more accurate comment explaining why.

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  vanilla:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: sudo apt-get update && sudo apt-get install -y make build-essential
+
+      - name: Build vanilla kernel
+        run: make vanilla
+
+  reproducibility:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: sudo apt-get update && sudo apt-get install -y make build-essential reprotest
+
+      - name: Run reprotest
+        # n.b. we don't run reprotest-sd because our kernels are not reproducible yet
+        # and also very slow. See <https://github.com/freedomofpress/kernel-builder/issues/3>.
+        run: make reprotest