[docker] build from desired tag in test-box

test-box always built `main` even when tagged, this
uses the tag as a version selector.

Author: fdupress

scripts/docker/Makefile

@@ -3,6 +3,7 @@
 # --------------------------------------------------------------------
 VARIANT ?= build
 TAG     ?= main
+BARGS   += --build-arg EC_VERSION=$(TAG)
 
 # --------------------------------------------------------------------
 .PHONY: default build publish
@@ -12,6 +13,7 @@ default: build
 build:
 	docker build -f Dockerfile.$(VARIANT) \
 	  --platform linux/amd64 \
+	  $(BARGS) \
 	  -t ghcr.io/easycrypt/ec-$(VARIANT)-box:$(TAG) \
 	  .