[docker] build setcap from alpine 3.9 (#1850)

There's an incompatibility between our version of git and Glide. It's
easier to downgrade our alpine build image (and thus git version) rather
than upgrade glide, as we have to move off of glide anyway.

docker/m3dbnode/Dockerfile-setcap

@@ -1,5 +1,5 @@
 # stage 1: build
-FROM golang:1.12-alpine AS builder
+FROM golang:1.12-alpine3.9 AS builder
 LABEL maintainer="The M3DB Authors <m3db@googlegroups.com>"
 
 # Install Glide