ci: init docker build workflow

.github/workflows/docker.yaml

@@ -0,0 +1,39 @@
+# https://github.com/actions/runner/blob/fde5227fbfe9/.github/workflows/publish-image.yml
+
+name: Build docker images
+
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - "k8s/github-runners/runner-image/**"
+
+env:
+  REGISTRY: ghcr.io
+
+jobs:
+  build:
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Docker buildx
+        uses: docker/setup-buildx-action@v3.8.0
+
+      - name: Login to ${{ env.REGISTRY }}
+        uses: docker/login-action@v2
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push runner image
+        uses: docker/build-push-action@v6.10.0
+        with:
+          context: ./k8s/github-runners/runner-image
+          tags: |
+            ${{ env.REGISTRY }}/${{ github.repository_owner }}/runner-image:${{ github.sha }}
+            ${{ env.REGISTRY }}/${{ github.repository_owner }}/runner-image:latest
+          push: true