Add Docker Github Action (#2086)

* Add Docker Github Action
- Build and upload arm64 + amd64 black images on push to master

This will need a `DOCKERHUB_USERNAME` and `DOCKERHUB_TOKEN` secrets set by someone with access.

* Change tag to push to pyfound/black repository. Thanks @ewdurbin

.github/workflows/docker.yml

@@ -0,0 +1,33 @@
+name: docker
+
+on:
+  push:
+    branches:
+      - "master"
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - 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: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: pyfound/black:latest
+
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}