Add CI workflow: lint, test, Docker build+push

ESLint, tsc type check, vitest, and Docker multi-stage build
with GHCR push on main merge. All jobs run on self-hosted ARC runners.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -0,0 +1,78 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  packages: write
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: cartsnitch/cartsnitch
+
+jobs:
+  lint:
+    runs-on: local-ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: npm
+      - run: npm ci
+      - name: ESLint
+        run: npx eslint .
+      - name: Type check
+        run: npx tsc --noEmit
+
+  test:
+    runs-on: local-ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: npm
+      - run: npm ci
+      - name: Run tests
+        run: npx vitest run
+
+  build-and-push:
+    runs-on: local-ubuntu-latest
+    needs: [lint, test]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to GHCR
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=sha,prefix=sha-
+            type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          target: prod