Properly scope Docker build flags for build-harness project

Makefile

@@ -5,27 +5,37 @@
 ifeq ($(BUILD_HARNESS_TOP_LEVEL_MAKEFILE_GUARD),)
 BUILD_HARNESS_TOP_LEVEL_MAKEFILE_GUARD := included
 
+########################################################################################
+## BEWARE: These variables are used by default by ALL PROJECTS that use Build Harness ##
+##         See the section below for variables that are specific to this project      ##
+########################################################################################
+
 export OS := $(shell uname -s | tr '[:upper:]' '[:lower:]')
 export BUILD_HARNESS_PATH ?= $(shell 'pwd')
 export BUILD_HARNESS_EXTENSIONS_PATH ?= $(BUILD_HARNESS_PATH)/../build-harness-extensions
 export BUILD_HARNESS_OS ?= $(OS)
 export BUILD_HARNESS_ARCH ?= $(shell uname -m | sed 's/x86_64/amd64/g')
 export SELF ?= $(MAKE)
 export PATH := $(BUILD_HARNESS_PATH)/vendor:$(PATH)
-# We do not have Alpine packages for arm64, so we need to force amd64 until we switch to Debian
-export DOCKER_BUILD_FLAGS ?= --platform linux/amd64
 
 # Forces auto-init off to avoid invoking the macro on recursive $(MAKE)
 export BUILD_HARNESS_AUTO_INIT := false
 
 # Debug should not be defaulted to a value because some cli consider any value as `true` (e.g. helm)
 export DEBUG ?=
 
+
+#############################################################################
+## SAFE: These variables are used only when building build-harness itself, ##
+##       so these are relatively safe to change                            ##
+#############################################################################
 ifeq ($(CURDIR),$(realpath $(BUILD_HARNESS_PATH)))
 # Only execute this section if we're actually in the `build-harness` project itself
 # List of targets the `readme` target should call before generating the readme
 export README_DEPS ?= docs/targets.md auto-label
 export DEFAULT_HELP_TARGET = help/all
+# We do not have Alpine packages for arm64, so we need to force amd64 until we switch to Debian
+export DOCKER_BUILD_FLAGS ?= --platform linux/amd64
 
 auto-label: MODULES=$(filter %/, $(sort $(wildcard modules/*/)))
 auto-label:
@@ -36,6 +46,8 @@ auto-label:
 # builder/build is defined in templates/Makefile.build-harness
 build: builder/build
 
+else
+export DOCKER_BUILD_FLAGS ?=
 endif
 
 # Import Makefiles into current context