Add hooks for multi-arch builds on dockerhub (#1683)

- hooks/build overrides the default build command to use buildx and
  create an image for amd64 and arm64 archictectures. The arm64 build
  will take time to build as it is emulated using qemu.
- hooks/push overrides the default push command to push the multi-arch
  images. The push is initiated from the buildx context since we have
  the push argument(--push) set in the buildx command.

docker/hooks/build

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+docker buildx create --name multiarch --use --platform linux/amd64,linux/arm64/v8
+docker buildx build ../ -t $IMAGE_NAME -f Dockerfile --platform linux/amd64,linux/arm64/v8 --push

docker/hooks/push

@@ -0,0 +1 @@
+#!/bin/bash