add a way to add plus version via make command (#7568)

Author: AlexFenlon

Makefile

@@ -2,7 +2,8 @@
 VER = $(shell grep IC_VERSION .github/data/version.txt | cut -d '=' -f 2)
 GIT_TAG = $(shell git describe --exact-match --tags || echo untagged)
 VERSION = $(VER)-SNAPSHOT
-PLUS_ARGS = --secret id=nginx-repo.crt,src=nginx-repo.crt --secret id=nginx-repo.key,src=nginx-repo.key
+NGINX_PLUS_VERSION ?= R33
+PLUS_ARGS = --build-arg NGINX_PLUS_VERSION=$(NGINX_PLUS_VERSION) --secret id=nginx-repo.crt,src=nginx-repo.crt --secret id=nginx-repo.key,src=nginx-repo.key
 
 # Variables that can be overridden
 REGISTRY                      ?= ## The registry where the image is located.