Makefile: Put container image and tag into variables

Makefile

+IMAGE=docker.io/library/alpine
+TAG=3.18
+
 image.sqfs:
-	podman run --rm -ti -v $$PWD:/build -w /build alpine:3.17 ./make-image.sh
+	podman run --rm --volume $$PWD:/build --workdir /build \
+		$(IMAGE):$(TAG) \
+		./make-image.sh
 
 .PHONY: image.sqfs