Skip the initial amd64-only Docker build (#11810)

PyNaCl's recent 1.5.0 release on PyPi includes arm64 wheels, which means our
arm64 docker images now build in a sensible amount of time, so we can skip the
amd64-only build.

.github/workflows/docker.yml

@@ -34,6 +34,8 @@ jobs:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
+      # TODO: consider using https://github.com/docker/metadata-action instead of this
+      # custom magic
       - name: Calculate docker image tag
         id: set-tag
         run: |
@@ -53,18 +55,6 @@ jobs:
           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:

changelog.d/11810.misc

@@ -0,0 +1 @@
+Docker: skip the initial amd64-only build and go straight to multiarch.