Add github actions for docker release. (#5146)

* Add github actions for docker release.

Once a release/tag shown up in GitHub,
an GitHub Action will be automatically triggered
for docker release.

This will help avoid additional steps when releasing.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update to address the review
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Address review comment
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

.github/workflows/docker.yml

+name: Docker Release
+
+on:
+  release:
+    types: [published]
+  workflow_dispatch:
+    inputs:
+      release:
+        description: "Release (e.g., v1.9.0)"
+        required: true
+
+jobs:
+  docker-release:
+    runs-on: ubuntu-latest
+    env:
+      DOCKER_LOGIN: ${{ secrets.DOCKERHUB_USERNAME }}
+      DOCKER_PASSWORD: ${{ secrets.DOCKERHUB_PASSWORD }}
+      RELEASE: ${{ github.event.inputs.release || github.event.release.tag_name }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build Docker Images
+        run: make VERSION=${RELEASE:1} DOCKER=coredns -f Makefile.docker release
+      - name: Show Docker Images
+        run: docker images
+      - name: Publish Docker Images
+        run: make VERSION=${RELEASE:1} DOCKER=coredns -f Makefile.docker docker-push