Move the docker image build to Github Actions (#10416)

it's flaky on circleCI, and having to manage multiple CI providers is painful.

.github/workflows/docker.yml

@@ -0,0 +1,72 @@
+# GitHub actions workflow which builds and publishes the docker images.
+
+name: Build docker images
+
+on:
+  push:
+    tags: ["v*"]
+    branches: [ master, main ]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up QEMU
+        id: qemu
+        uses: docker/setup-qemu-action@v1
+        with:
+          platforms: arm64
+
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Inspect builder
+        run: docker buildx inspect
+          
+      - name: Log in to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Calculate docker image tag
+        id: set-tag
+        run: |
+          case "${GITHUB_REF}" in
+              refs/heads/master|refs/heads/main)
+                  tag=latest
+                  ;;
+              refs/tags/*)
+                  tag=${GITHUB_REF#refs/tags/}
+                  ;;
+              *)
+                  tag=${GITHUB_SHA}
+                  ;;
+          esac
+          echo "::set-output name=tag::$tag"
+
+        # for release builds, we want to get the amd64 image out asap, so first
+        # we do an amd64-only build, before following up with a multiarch build.
+      - name: Build and push amd64
+        uses: docker/build-push-action@v2
+        if: "${{ startsWith(github.ref, 'refs/tags/v' }}"
+        with:
+          push: true
+          labels: "gitsha1=${{ github.sha }}"
+          tags: "matrixdotorg/synapse:${{ steps.set-tag.outputs.tag }}"
+          file: "docker/Dockerfile"
+          platforms: linux/amd64
+
+      - name: Build and push all platforms
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          labels: "gitsha1=${{ github.sha }}"
+          tags: "matrixdotorg/synapse:${{ steps.set-tag.outputs.tag }}"
+          file: "docker/Dockerfile"
+          platforms: linux/amd64,linux/arm64