GitHub action for automatic multiarch Docker build (#1613)

* Create docker-build-and-push.yml

* Switched to using official Docker buildx action

* Added comment to docker-build-and-push.yml
In case something needs to be tweaked in the future, the link
contains some examples and explanations which would be useful

* Run only on release (and produce release tags)

As this workflow takes quite a lot of time, and [pushing to master happens frequently](https://github.com/matrix-org/dendrite/pull/1613#issuecomment-746086980), the container will now only be built when a release is created, and the builds will also be correctly tagged.

* Add latest tag, test at neilalexander/dendrite*

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

.github/workflows/docker-build-and-push.yml

@@ -0,0 +1,73 @@
+# Based on https://github.com/docker/build-push-action
+
+name: "Docker Multiarch Build & Push"
+
+on:
+  release:
+#    types: [published]
+    branches: [master]
+
+env:
+  DOCKER_HUB_USER: neilalexander
+
+jobs:
+  BuildAndPush:
+    runs-on: ubuntu-latest
+    steps:
+      - 
+        name: Checkout
+        uses: actions/checkout@v2
+      - 
+        name: Get release tag
+        run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+      -
+        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: ${{ env.DOCKER_HUB_USER }}
+          password: ${{ secrets.DOCKER_TOKEN }}
+      -
+        name: Build temporary (builder) image
+        id: docker_build_temporary
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./build/docker/Dockerfile
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
+          push: false
+          tags: ${{ env.DOCKER_HUB_USER }}/dendrite:latest
+      -
+        name: Build monolith image
+        id: docker_build_monolith
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./build/docker/Dockerfile.monolith
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
+          push: true
+          tags: |
+            ${{ env.DOCKER_HUB_USER }}/dendrite-monolith:latest
+            ${{ env.DOCKER_HUB_USER }}/dendrite-monolith:${{ env.RELEASE_VERSION }}
+      -
+        name: Build polylith image
+        id: docker_build_polylith
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./build/docker/Dockerfile.polylith
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
+          push: true
+          tags: |
+            ${{ env.DOCKER_HUB_USER }}/dendrite-polylith:latest
+            ${{ env.DOCKER_HUB_USER }}/dendrite-polylith:${{ env.RELEASE_VERSION }}
+      -
+        name: Image digest
+        run: |
+          echo Monolith ( ${{ env.RELEASE_VERSION }} ) image digest - ${{ steps.docker_build_monolith.outputs.digest }}
+          echo Polylith ( ${{ env.RELEASE_VERSION }} ) image digest - ${{ steps.docker_build_polylith.outputs.digest }}