CI: add Docker build workflow

.github/workflows/docker-build.yml

@@ -0,0 +1,28 @@
+name: Docker build
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      # Build only (no push). If this fails, the PR breaks container builds.
+      - name: Build Docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          push: false
+          tags: clawdbot-railway-template:ci
+          cache-from: type=gha
+          cache-to: type=gha,mode=max