Add github workflow

Automatically build for amd64 and arm64

.github/workflows/main.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: CI
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  build:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+    # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+    - uses: actions/checkout@v2
+
+    # Runs a single command using the runners shell
+    - name: Build and Push Docker Image (Linux - AMD64)
+      uses: docker/build-push-action@v1
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
+        repository: ekofr/pihole-exporter
+        tag_with_sha: true
+        tags: amd64
+        build_args: OS=linux,ARCH=amd64
+    - name: Build and Push Docker Image (Linux ARM64)
+      uses: docker/build-push-action@v1
+      with:
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
+        repository: ekofr/pihole-exporter
+        tag_with_sha: true
+        tags: armv64
+        build_args: OS=linux,ARCH=arm64