feat: sync Cloudflare DNS records with Traefik container hosts

Create a record for each Host(...) in running containers' Traefik router
labels, tagged with a configurable comment and optional tags, and delete
owned records once their host has been down for DELETE_AFTER.

.github/workflows/ci.yml

@@ -0,0 +1,54 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*.*.*"]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+      - run: test -z "$(gofmt -l .)"
+      - run: go vet ./...
+      - run: go test -race ./...
+
+  image:
+    needs: test
+    if: github.event_name == 'push'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v5
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=edge,branch=main
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}