docker: test makefile and Dockerfile change (#4866)

The fix in 4b9bc138d9 breaks docker with

~~~
Step 2/9 : RUN sed -i.bak s@stable/update@stable-security/update@g /etc/apt/sources.list
 ---> Running in 29a0dbae5746
no status provided on response: unknown
~~~

So reverted here. This adds github workflows to test Makefile.docker
syntax as well. And small updates to the docker-coredns over in the
release repo.

Signed-off-by: Miek Gieben <miek@miek.nl>

Makefile.docker

@@ -68,7 +68,7 @@ else
 	@# 1. Copy appropriate coredns binary to build/docker/linux/<arch>
 	@# 2. Copy Dockerfile to build/docker/linux/<arch>
 	for arch in $(LINUX_ARCH); do \
-	    docker build -t $(DOCKER_IMAGE_NAME)-$${arch}:$(VERSION) build/docker/linux/$${arch} ;\
+	    docker build -t $(DOCKER_IMAGE_NAME)-$${arch}:$(VERSION) build/docker/$${arch}  && \
 	    docker tag $(DOCKER_IMAGE_NAME)-$${arch}:$(VERSION) $(DOCKER_IMAGE_NAME)-$${arch}:latest ;\
 	done
 endif