Use build container id rather than tagging builds

This means that multiple builds will not conflict, so we can
remove the lock from the CI. Also quieter when no errors.

Some still left to do, only done the ones used in build and CI
initially. Some of the others will be cleaned up anyway later.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/packages/iptables/Makefile

@@ -2,8 +2,9 @@ all: usr/local/sbin/iptables
 
 usr/local/sbin/iptables: Dockerfile main.ml
 	mkdir -p usr/local/sbin
-	docker build -t iptables:build .
-	docker run --rm iptables:build | tar xf - -C usr/local/sbin
+	BUILD=$$( docker build -q . ) && [ -n "$$BUILD" ] && \
+	docker run --rm $$BUILD | tar xf - -C usr/local/sbin && \
+	docker rmi --no-prune $$BUILD
 
 clean:
 	rm -rf usr