use GitHub Actions to build Docker container (#1181)

Start using GitHub Actions to build the Docker container for this
project. Builds all tags and master as the latest container tag. Builds
for amd64 and arm64.

.github/workflows/docker.yml

@@ -0,0 +1,45 @@
+name: docker
+
+on:
+  push:
+    branches:
+      - master
+    tags:
+      - '*'
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: docker/setup-qemu-action@v1
+
+      - uses: docker/setup-buildx-action@v1
+
+      - id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: mher/flower
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+
+      - uses: docker/login-action@v1
+        if: github.event_name != 'pull_request'
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - uses: docker/build-push-action@v2
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}