ci: add in build process

.github/workflows/build.yml

@@ -0,0 +1,49 @@
+name: build
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - '^\d+\.\d+\.\d+$'
+  schedule:
+    - cron: '0 0 * * * *'
+
+jobs:
+  multi:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
+      -
+        name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      -
+        name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Inject slug/short variables
+        uses: rlespinasse/github-slug-action@v3.x
+
+      - if: github.ref === 'refs/heads/main'
+        run: echo ::set-output name=tag::latest
+      - if: startsWith(github.ref, 'refs/tags/')
+        run: echo ::set-output name=tag::${GITHUB_REF#refs/*/}
+
+      - if: env.tag
+        name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/386,linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/ppc64le,linux/s390x
+          push: true
+          tags: |
+            willfarrell/crontab:${{ env.tag }}