Build binaries with CGO disabled to support non-desktop environments

Right now, the binary [releases](https://github.com/cloudflare/cloudflared/releases) are the only official distribution for non-amd64 architectures. They are compiled with implicit `CGO_ENABLED=1`, which dynamically links them to libc. This is a problem not only for Docker (#221), but also for Alpine, which doesn't use libc. Looking at your existing Docker images, they are [already built](https://github.com/cloudflare/cloudflared/blob/0929e5f0ff40c69d15bb9bd395db81e9f14143db/Dockerfile#L6) with explicit `CGO_ENABLED=0`. Can you please make the same changes to the binary releases? I have tested manually building such releases and they work just fine. This will bring support to a lot more environments. Thanks.