Add CGO_ENABLED=0 to build

Turns out, building on debian and running on alpine does not work, if CGO is enabled, because the binary will be linked against glibc, and alpine uses musl libc instead. I don’t think we’re using any features, that depend on libc (or rather nothing the alternative go implementation doesn’t offer, mostly nsswitch stuff), so I disabled CGO in the build. Ths produces a statically linked binary and we can still use sweet sweet 5 MB alpine as base image for running.