The variable was set in the Makefile, but set as an empty string

Makefile

@@ -1,3 +1,7 @@
+# It'd be nice to keep these in sync with the defaults of the Dockerfiles
+PYTHON_IMAGE_VER ?= 3.12
+POSTGRES_IMAGE_VER ?= 12.3
+
 .DEFAULT_GOAL := help
 
 ## toggle-prod: configure make to use the production stack
@@ -37,7 +41,7 @@ behave-translator: compose.override.yml
 ## build: rebuilds all your containers or a single one if CONTAINER is specified
 .Phony: build
 build: compose.override.yml
-	@docker compose build --build-arg PYTHON_IMAGE_VER=${PYTHON_IMAGE_VER} --build-arg POSTGRES_IMAGE_VER=${POSTGRES_IMAGE_VER} $(CONTAINER)
+	@docker compose build --build-arg PYTHON_IMAGE_VER=$(PYTHON_IMAGE_VER) --build-arg POSTGRES_IMAGE_VER=$(POSTGRES_IMAGE_VER) $(CONTAINER)
 	@docker compose up -d --no-deps $(CONTAINER)
 	@docker compose restart $(CONTAINER)