Add support for multi platform image

Makefile

@@ -24,6 +24,7 @@ SHELL = /usr/bin/env bash -o pipefail
 GIT_COMMIT_SHA ?= "$(shell git rev-parse HEAD 2>/dev/null)"
 GIT_TAG ?= $(shell git describe --tags --dirty --always)
 PLATFORMS ?= linux/amd64
+PUBLISH_PLATFORMS ?= linux/amd64,linux/arm64
 DOCKER_BUILDX_CMD ?= docker buildx
 IMAGE_BUILD_CMD ?= $(DOCKER_BUILDX_CMD) build
 IMAGE_BUILD_EXTRA_OPTS ?=
@@ -66,6 +67,12 @@ SYNCER_IMAGE_BUILD_EXTRA_OPTS += -t $(SYNCER_IMAGE_EXTRA_TAG)
 BBR_IMAGE_BUILD_EXTRA_OPTS += -t $(BBR_IMAGE_EXTRA_TAG)
 endif
 
+# Allow `make MULTI=true ...` in CI to switch to multi-arch.
+ifdef MULTI
+  PLATFORMS := $(PUBLISH_PLATFORMS)
+endif
+
+
 # The name of the kind cluster to use for the "kind-load" target.
 KIND_CLUSTER ?= kind
 
@@ -193,7 +200,7 @@ image-build: ## Build the EPP image using Docker Buildx.
 
 .PHONY: image-push
 image-push: PUSH=--push ## Build the EPP image and push it to $IMAGE_REPO.
-image-push: image-build
+image-push: MULTI=true image-build
 
 .PHONY: image-load
 image-load: LOAD=--load ## Build the EPP image and load it in the local Docker registry.