Github Action to build docker image of knative mkdocs (#4464)

.github/workflows/knative-mkdocs-dockerimage.yaml

@@ -0,0 +1,30 @@
+name: Docker Image CI
+
+on:
+  push:
+    branches: ['main']
+
+jobs:
+  build-and-push-image:
+    name: Build knative mkdocs dockerfile
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push docker image
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          file: ./hack/docker/Dockerfile
+          tags: ghcr.io/knative/knative-docs:latest