feat: use docker buildx in order to build a multi-arch docker image

Note: this also automagically pushes the images using buildx as they are
not imported to the actual docker system anymore.

Signed-off-by: Matthias Riegler <matthias.riegler@traefik.io>

build-functions/docker-functions.sh

@@ -1,8 +0,0 @@
-#!/bin/bash
-
-push_image_to_registry() {
-  local target_tag=$1
-  echo "⏫ Pushing '${target_tag}'"
-  $DRY docker push "${target_tag}"
-  echo "✅ Finished pushing the Docker image '${target_tag}'."
-}