Add devcontainer CI workflow

Boots the devcontainer on a monthly schedule (and on manual dispatch) to
catch regressions in the dev environment — Oracle Instant Client URLs,
gvenzl/oracle-free image changes, and base image drift — before they hit
contributors. Runs the spec suite and rubocop inside the booted container.

Author: yahonda

.github/workflows/devcontainer.yml

@@ -0,0 +1,28 @@
+name: devcontainer
+
+on:
+  schedule:
+    - cron: "0 6 1 * *"  # 1st of every month at 6:00 AM UTC
+  workflow_dispatch:
+
+env:
+  RUBYOPT: "-w"
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Ensure .bundle mount source exists
+        run: mkdir -p .bundle
+
+      - name: Boot dev container and run tests
+        uses: devcontainers/ci@v0.3
+        with:
+          runCmd: |
+            uname -a
+            lsb_release -a
+            ruby --version
+            bundle exec rake spec
+            bundle exec rubocop