Switch to alpine for all platforms

[tomdee]

alpine has packages for iptables (https://pkgs.alpinelinux.org/packages?name=iptables&branch=&repo=&arch=&maintainer=)

Hopefully there are also apine base images for all platforms - e.g. https://hub.docker.com/r/arm32v6/alpine/

[tomdee]

...but this isn't going to be easy. The udp backend needs CGO which means a full cross-compiling C environment is needed. kube-cross provides this but it produces binaries linked against glibc which won't run on alpine unless glibc packages are installed. And glibc for alpine is only (easily) available for amd64.

I wasn't able to get kube-cross to link against musl libc and I wasn't able to make an alpine/musl based cross compiling build container.

So I'm giving up on this for now...

[tomdee]

Fixed by #860