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>

Dockerfile

@@ -1,6 +1,5 @@
 FROM debian:stable-slim
 
-RUN sed -i.bak s@stable/update@stable-security/update@g /etc/apt/sources.list
 RUN apt-get update && apt-get -uy upgrade
 RUN apt-get -y install ca-certificates && update-ca-certificates