Add docker image DockerHub workflow

This workflow automates the generation of Docker images and publishes them
to DockerHub as necessary.

Signed-off-by: Jeremy Ho <jujaga@gmail.com>

Author: jujaga

.github/workflows/docker-image.yaml

@@ -0,0 +1,69 @@
+---
+name: Docker Image CI
+on:
+  push:
+    branches:
+      - master
+    tags:
+      - "v*"
+  pull_request:
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: docker
+    outputs:
+      HAS_DOCKER_SECRETS: ${{ steps.check-secrets.outputs.HAS_DOCKER_SECRETS }}
+    timeout-minutes: 10
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Check Workflow Secrets
+        id: check-secrets
+        run: |
+          echo "::set-output name=HAS_DOCKER_SECRETS::${{ secrets.DOCKERHUB_USERNAME != '' && secrets.DOCKERHUB_TOKEN != '' }}"
+      - name: Build the Docker image
+        run: docker build . --file Dockerfile --tag certbot:$(date +%s)
+  docker-publish:
+    needs: docker-build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: docker
+    timeout-minutes: 10
+    if: github.event_name != 'pull_request' && needs.docker-build.outputs.HAS_DOCKER_SECRETS == 'true'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: bcgovimages/certbot
+          # Creates tags based off of branch names and semver tags
+          tags: |
+            type=schedule
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha
+          # Always updates the 'latest' tag
+          flavor: |
+            latest=true
+      - name: Build and Push to DockerHub
+        uses: docker/build-push-action@v2
+        with:
+          context: docker
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+      - name: Inspect Docker Image
+        run: docker image inspect bcgovimages/certbot:latest