Create workflow for testing for the ruby feature

This workflow spins up a devcontainer that uses the ruby feature and checks that ruby is installed.

.github/.devcontainer/devcontainer.json

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+{
+  "image": "mcr.microsoft.com/devcontainers/base:1-bookworm",
+  "features": {
+    "ghcr.io/devcontainers/features/common-utils:2": {
+      "installZsh": "true",
+      "username": "vscode",
+      "userUid": "1000",
+      "userGid": "1000",
+      "upgradePackages": "true"
+    },
+    "ghcr.io/devcontainers/features/git:1": {
+      "version": "latest",
+      "ppa": "false"
+    },
+    "./features/ruby": {}
+  },
+  // Set `remoteUser` to `root` to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
+  "remoteUser": "vscode"
+}

.github/.devcontainer/features

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+../../features

.github/workflows/test-ruby-feature.yaml

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+name: Test ruby feature
+
+on:
+  push:
+
+jobs:
+  build:
+    name: Test ruby feature
+
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout (GitHub)
+        uses: actions/checkout@v3
+
+      - uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and test devcontainer
+        uses: devcontainers/ci@v0.3
+        with:
+          imageName: ghcr.io/rails/devcontainer/test-ruby-feature
+          cacheFrom: ghcr.io/rails/devcontainer/test-ruby-feature
+          subFolder: .github
+          refFilterForPush: refs/heads/main
+          runCmd: ruby -v || ( echo "Ruby is not installed."; exit 1 )