Add GitHub workflow file

This replaces our Semaphore CI setup.

.github/workflows/linux.yml

@@ -0,0 +1,11 @@
+name: Linux CI
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build the test image
+        run: docker build -t popt -f ci/Dockerfile .
+      - name: Run the test suite
+        run: docker run -t popt