Add GitHub docker workflow

Author: satterly

.github/FUNDING.yml

@@ -0,0 +1 @@
+github: satterly

.github/workflows/docker.yml

@@ -0,0 +1,40 @@
+name: Docker Build and Push
+
+on:
+  push:
+    branches: [ master, release/* ]
+
+env:
+  SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      REPOSITORY_URL: docker.pkg.github.com
+      IMAGE_NAME: ${{ github.repository }}/alerta-cli
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Variables
+        id: vars
+        run: echo "::set-output name=SHORT_COMMIT_ID::$(git rev-parse --short HEAD)"
+      - name: Build image
+        run: >-
+          docker build
+          -t $IMAGE_NAME
+          -t $REPOSITORY_URL/$IMAGE_NAME:${{ steps.vars.outputs.SHORT_COMMIT_ID }}
+          -t $REPOSITORY_URL/$IMAGE_NAME:latest .
+      - name: Docker Login
+        env:
+          DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
+          DOCKER_PASSWORD: ${{ secrets.GITHUB_TOKEN }}
+        run: docker login $REPOSITORY_URL --username "$DOCKER_USERNAME" --password "$DOCKER_PASSWORD"
+      - name: Publish Image
+        run: docker push $REPOSITORY_URL/$IMAGE_NAME
+
+      - name: Notify Slack
+        uses: alerta/actions/slacky@master
+        with:
+          status: ${{ job.status }}
+        if: always()