Add rudimentary devcontainer CI configuration

.github/workflows/test_devcontainers.yaml

@@ -0,0 +1,18 @@
+name: 'Test devcontainers'
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout (GitHub)
+        uses: actions/checkout@v4
+
+      - name: Build and run dev container task
+        uses: devcontainers/[email protected]
+        with:
+          runCmd: opensafely run run_all