Update GHA Docker Hub builds

Squashed commit of the following:

commit 4a61aa711473deece2adf415cfd65501dbca63b2
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Mon Jan 18 12:19:24 2021 +0000

    Set back to matrixdotorg on published releases

commit 6d1ac53f2c0c9b30e1e70c0bb1559e1b8ec874a2
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Mon Jan 18 11:55:28 2021 +0000

    Rename

commit 258999f7fb7b655b3a02a06a7ea05e66fb7740fb
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Mon Jan 18 11:52:26 2021 +0000

    Refactor multi-stage builds

commit c7ab8e476939899571e7b5668860dec372b9b60f
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Mon Jan 18 11:13:19 2021 +0000

    Let's try this again

1 files changed, 34 insertions, 37 deletions

diff --git a/.github/workflows/docker-build-and-push.yml b/.github/workflows/docker-hub.yml
index 85aa0ab5..84745f7b 100644
--- a/.github/workflows/docker-build-and-push.yml
+++ b/.github/workflows/docker-hub.yml
@@ -1,73 +1,70 @@
 # Based on https://github.com/docker/build-push-action
 
-name: "Docker Multiarch Build & Push"
+name: "Docker Hub"
 
 on:
-  push:
-#    types: [published]
-    branches: [docker]
+  release:
+    types: [published]
 
 env:
-  DOCKER_HUB_USER: neilalexander
+  DOCKER_HUB_USER: matrixdotorg
+  PLATFORMS: linux/amd64,linux/arm64,linux/arm/v7
 
 jobs:
-  BuildAndPush:
+  Monolith:
     runs-on: ubuntu-latest
     steps:
-      - 
-        name: Checkout
+      - name: Checkout
         uses: actions/checkout@v2
-      - 
-        name: Get release tag
+      - name: Get release tag
         run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
-      -
-        name: Set up QEMU
+      - name: Set up QEMU
         uses: docker/setup-qemu-action@v1
-      -
-        name: Set up Docker Buildx
+      - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
-      -
-        name: Login to DockerHub
+      - name: Login to Docker Hub
         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
+
+      - 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
+          platforms: ${{ env.PLATFORMS }}
           push: true
           tags: |
             ${{ env.DOCKER_HUB_USER }}/dendrite-monolith:latest
             ${{ env.DOCKER_HUB_USER }}/dendrite-monolith:${{ env.RELEASE_VERSION }}
-      -
-        name: Build polylith image
+
+  Polylith:
+    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 Docker Hub
+        uses: docker/login-action@v1 
+        with:
+          username: ${{ env.DOCKER_HUB_USER }}
+          password: ${{ secrets.DOCKER_TOKEN }}
+
+      - 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
+          platforms: ${{ env.PLATFORMS }}
           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 }}